[Epic] reimplement user profile tabs and feed

[brylie]

🚧 This epic is a placeholder for several tasks that have not yet been defined. 🚧

Goal

Starting from a more straightforward project footprint, we want to rebuild some features that were removed to deprecate the project Backbone JavaScript. While the initial pull request removes some features from the accounts app, we will use this epic to re-implement the features using only Django.

A sub-goal will be to unify the user profile views so we can share code, such as the UserCivis page displaying Civis for a specific user while letting that user also access their Civi management features if it is their profile.

Idea summary

• Reimplement the user profile tabs ("My Civis", "Followers", "Following", and "My Issues")
• Reimplement follow/unfollow feature

Further details

The user profile tabs were removed as part of #1428 to simplify the development process. However, they are helpful features that we should consider adding again.

Task

• Create a base template used for all UserProfile sub-views #1452
• Add "Civis" tab to the user profile view #1445
• Add "Followers" tab to the user profile view #1446
• Add "Following" tab to the user profile view #1447
• Add "Issues" tab to the user profile view #1448
• Add "Activity" tab to the user profile view #1449
• Reimplement "Follow/Unfollow" feature on user profile view
• Reimagine user Feed view #1450
• Reimplement user categories feature #1451