Add Individual users to Bubble Timeline

[ch0ccyra1n]

Pitch

I think it would be great to have an option to add individual users to the bubble timeline, in addition to entire instances.

Related: #4 (comment)

Motivation

For example, @ Eugenecriticalmass is relevant to my instance's community, but I do not want to add all of mstdn.social to the bubble timeline, just this one user. There are certainly other examples of situations like this, where one might want to add just a single user to the bubble instead of their entire instance.