(Optionally) Warn when inviting users who might not exist.

[lampholder]

We made a change to the invite UX to stop people mistakenly inviting 'nonexistant' mxids.

The change relied on checking a user's /profile to establish whether they exist or not, but it's become clear that this isn't always appropriate - particulary in the case of bridged users.

A spec change is being discussed as to the merits of checking /profile and how we might enhance this API (or add a new one) to handle checking whether a user exists. However, we've decided that for now we can power through client side if we:

• expose a warning when you try and invite a mxid we can't confirm exists (but let you continue with the invite if you wish)
• make this warning optional, to be configured on either a per user or per riot deploy basis

[lampholder]

Closing in favour of #7922 (where I should have written the above)