Pixelfed: Allow for sorting and categorizing activities

[hellcp]

@RavengamerSpace had a very good idea of seperating activities into categories. While 3 column system might not be a good idea, I think we need to have a way to see only likes, mentions or follows

[Aurnytoraink]

Here is a example:

[dansup]

@RavengamerSpace I think thats an interesting idea, I think a js filter would be more appropriate so you keep the chronological aspect in a single column

[dansup]

You can now filter notifications by activity type!