[server] core: Replace flask-caching dependency with drop-in

[ix5]

Checklist

• All new and existing tests are passing
• (If adding features:) I have added tests to cover my changes
• (If docs changes needed:) I have updated the documentation accordingly.
• I have added an entry to CHANGES.rst because this is a user-facing change or an important bugfix
• I have written proper commit message(s)
  

What changes does this Pull Request introduce?

core: Replace flask-caching dep with drop-in

Instead of relying on flask-caching and its dep cachelib, import a trimmed-down version into the repo as utils/cache.py.

Why is this necessary?

While seeing pallets-eco/flask-caching#352 that currently breaks Isso, it became apparent that Flask-Caching has grown way too large and has gained dependencies of its own.

Initially, Isso depended on SimpleCache and NullCache from werkzeug.contrib, which meant that it didn't have to import any new dependencies. In a later version, caching was removed from werkzeug and instead pushed into its own package, which Isso then had to import.
With the newest release of Flask-Caching, there's now a hard dependency on cachelib as well, in addition to flask.

Isso only uses get(), set() and delete() cache functions and only for email/website hashes. We can drop all those dependencies and instead import a slimmed-down version on our own.