Make all four corners of the Contribution Banner 3.0 rounded

[zenparsing]

Resolves brave/brave-browser#30282
Resolves brave/brave-browser#30331
Resolves brave/brave-browser#30489

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run lint, npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[emerick]

LGTM

[mkarolin]

LGTM

[LaurenWags]

Verified with

Brave | 1.53.60 Chromium: 114.0.5735.26 (Official Build) nightly (x86_64)
-- | --
Revision | 7075cbb66f0542ac3e01ddfde6b813e7d61118a5-refs/branch-heads/5735@{#454}
OS | macOS Version 13.4 (Build 22F66)

Issue 1 - brave/brave-browser/issues/30331 - PASSED

1. Be logged in to twitter
2. Have rewards enabled and connected to a custodian
3. Navigate to a creator who is verified with same custodian as you
4. Make a contribution
5. On success banner, select "Share your support"
6. View pre-canned tweet on twitter
   --> Confirmed addition of "BAT" to pre-canned tweet:

Issue 2 - brave/brave-browser/issues/30489 - PASSED

Confirmed all 4 corners of contribution (tip) banner are rounded:

Dark	Light

[LaurenWags]

Verified with

Brave	1.53.60 Chromium: 114.0.5735.26 (Official Build) nightly (64-bit) 
Revision	7075cbb66f0542ac3e01ddfde6b813e7d61118a5-refs/branch-heads/5735@{#454}
OS	Windows 10 Version 21H2 (Build 19044.2965)

Issue 2 - brave/brave-browser/issues/30489 - PASSED

Confirmed all 4 corners of contribution (tip) banner are rounded:

Dark	Light

[LaurenWags]

Verified with

Brave	1.53.60 Chromium: 114.0.5735.26 (Official Build) nightly (64-bit) 
Revision	7075cbb66f0542ac3e01ddfde6b813e7d61118a5-refs/branch-heads/5735@{#454}
OS	Linux

Issue 2 - brave/brave-browser/issues/30489 - PASSED

Confirmed all 4 corners of contribution (tip) banner are rounded:

Dark	Light