Entry tags/metadata redux

[lemon24]

The unified tag/metadata API introduced in #266 makes it relatively easy to support entry tags, and more and more use cases keep cropping up ... so, it's finally time!

Global tags will be very similar to implement, doing it in this issue seems like a good idea.

I'm postponing filtering entries by entry tags for now, since I don't need it yet, and it requires additional refactoring.

For reference, here's the list of use cases we have until now:

• user notes, #228
• post reactions, #228
• enclosure download info, #228
• entry read time (via plugin)
• how far the user has read an entry, https://github.com/lemon24/reader/issues/271#issuecomment-1045103819
• dedupe id (although maybe this should be a core attribute), https://github.com/lemon24/reader/issues/271#issuecomment-1045103819
• application configuration, #267

[lemon24]

To do:

• [x] tables, migration
• [x] Reader methods (skip None and ('feed', None) wildcards for now)
• [x] tests
• [x] entry_dedupe support
• [x] deprecate global_metadata (move to examples)
• [x] allow editing metadata through the web app
• [x] docs
  • [x] get_tags(wildcard) deprecation
  • [x] changelog
  • [x] user guide
  • [x] dev notes
  • [x] docstrings, versionchanged