Username autocompletion

[chucker]

Suggest users when typing @ followed by part of a user name or display name.

Multiple aspects required:

• we need a UI like Gitter's own, or like in https://github.com/Blazored/Typeahead/ (but we can't outright use Typeahead, I believe, because it seems to always require a drop-down rather than being invoked inline from the message input box). Stuff like arrow up/down to select someone, etc.

• we need JS interop to get the cursor position (I think)

• we need the API-side search. Unlike Gitter's own client, this PR takes advantage of #75 so results are more comprehensive.

[chucker]

A very early version of the UI is now here. The positioning is, too, but doesn't seem to actually work yet.

[chucker]

I've removed draft status because this is way beyond proof of concept now.

Remaining issues before this should be merged:

• add a way to select a suggested user and autocomplete the input box that selection. Probably up/down arrow keys, then return. Maybe also clicking/tapping?

Remaining issues that I don't think should block a merge:

• the cache doesn't discriminate by room. If you're in multiple rooms, it'll suggest users that aren't actually here. This is technically a design flaw, but I'm not sure it matters much in practice — you probably know whom you want to @. I can't think of a way to rectify this short of significantly complicating the data structure.
• the UI isn't pretty yet.
• horizontal alignment doesn't work. I feel like I'm missing a stupidly obvious way here; I can only think of a measureString approach (measure the text up until before the @, then left-align the popup there). That would work, but seems overcomplicated?

[SQL-MisterMagoo]

For the user list, I would personally be happy to let it pop up where you have it - the horizontal doesn't matter too much, but if you want to try and position it, I found this that seems to work https://medium.com/@jh3y/how-to-where-s-the-caret-getting-the-xy-position-of-the-caret-a24ba372990a

[chucker]

if you want to try and position it, I found this that seems to work https://medium.com/@jh3y/how-to-where-s-the-caret-getting-the-xy-position-of-the-caret-a24ba372990a

Yeah, the offscreen div approach is one I evaluated. I'll consider it.