Gutenberg-mobile: Support @-mentions while editing

[hypest]

It would be nice if we could support @-mentions while editing, to add an explicit mention to a site user.

[koke]

Just adding that rather than mentions specifically, Gutenberg supports a customizable system of autocompleters, so whenever we implement this we should build on top of that.

[designsimply]

This came up in a mobile request from @elibud after it was reported in some feedback that @ mentions were not working in expanded view for comments on WPiOS (now fixed) and we did a full audit of mentions after that. Username mentions are expected to work while composing a post and because this is an important feature when working remotely, can we move this request up in prioritization? @hypest can you help me with that by letting me know what release we can try for to get this added to?

^{(internal reference: p4a5px-2vE-p2)}

[hypest]

@SergioEstevao will do a spike on this one to see what are the pieces missing/etc. This feature is probably also going to need some design (headsup @iamthomasbishop).

No specific target version yet @designsimply . We need to uncover more info before being specific there.

[elibud]

I'd like to see this one prioritized please.

[hypest]

@SergioEstevao is a bit busy at the moment and will be on some rotations the following 2 weeks so, @mchowning will step in next week to pick up this feature.

[mchowning]

:wave: @iamthomasbishop ! Do you have an svg icon to be used for the button?

[iamthomasbishop]

@mchowning Assuming you mean the @ symbol, I don't at the moment but I will create one today.

[iamthomasbishop]

As mentioned in DM, icon was added to the design docs.