Autocomplete when inviting users implies nonexistent mxids exist

[ara4n]

We should probably only show a nice @.......:........ box if we've got a profile response for a given mxid, otherwise if you typo it hints to the user that the mxid is valid when it isn't.

[lampholder]

I'm sure we've discussed this elsewhere, but for any given mxid entered into the invite box, there are three states:

• Valid and recognised: Riot found some profile information for this mxid and can turn the entry in the invite list into a nice solid reassuring pill. Pillificaiton should only be done after the user has definitely finished typing the mxid, to avoid pillifying @ user:example.co when they were reaching for @ user:example.com.
• Valid and unrecognised: Riot found no profile information for this mxid. We should turn it into a pill, but one with an indicator of uncertainty. A question mark avatar, grey background.
• Invalid: @ josé:matrix.org: valid mxids only have ascii characters in them, so don't turn this into even an uncertain pill - maybe give it wiggly underlines or something.