FIX: revert previously removed mentions transformation on the client

[AndrewPrigorshnev]

This partially reverts 2ecc829.

The problem is that if we don't transform mentions right away, there is a noticeable lag before a mention gets fully rendered:

Screen.Recording.2023-08-14.at.19.00.22.mov

even when running locally:

Screen.Recording.2023-08-14.at.18.56.07.mov

While with this transformation, everything is super smooth:

Screen.Recording.2023-08-14.at.18.56.40.mov

I'm reverting that change only for mentions. Another part was about category hashtags, but unfortunately they lag both with and without this transformation. We need to address them separately.

cc @martin-brennan

[jjaffeux]

Will let @martin-brennan look at this one given he changed this code few days ago

[martin-brennan]

Ah my bad @AndrewPrigorshnev I didn't read properly and see simpleCategoryHashMentionTransform was doing mentions too 🤦 As for the hashtag render lag, I will have to have a think about it. We already do something like this in the composer preview, but I know that needs to do a /hashtags/lookup API call and we don't want to introduce this overhead here. At the very least I could just bring back the old code and only make it render the grey placeholder with the square next to it, but not actually make it into a URL etc. . I don't want something that is too much of an outlier from the rest of the hashtag code. Will tag you in the review when I do it 👍

[AndrewPrigorshnev]

@martin-brennan, great, thank you for looking into the hashtag thing! And could we merge this part for mentions first? May I have an approval on it?

[martin-brennan]

@AndrewPrigorshnev so sorry I didn't realise I didn't approve 🤦 Go ahead and merge!

[AndrewPrigorshnev]

@martin-brennan, no worries! Thank you for the review and the approval.