Add ability to bulk-import subreddits

[aouerf]

The first, and probably the most proper method, is to set up user authentication so I can retrieve a list of subreddits directly from the user. Unfortunately this is impractical at the moment since no system is set up in place for it.

The second option is to ask the user to download a list of their subreddits through Reddit as a JSON file and import them in the app. Unfortunately, this seems to be capped at 100 subreddits per page, so if the user wants to import more they would have to deal with pagination and multiple files, which isn't ideal.

The third option, is to read entries from a text file, where each entry can be a full subreddit URL (e.g. https://www.reddit.com/r/redditdev) or simply the subreddit name (e.g. r/redditdev or redditdev). This is by far the easiest to implement, but comes with the caveat that files have to be formatted a certain way, and file validation have to be handled properly.

[aouerf]

Another thing worth noting is that, starting from Android 10, notifications are capped to 25 per app (was 50 before).