Simplify Gothic to use our session store instead of creating a different store (...

Simplify Gothic to use our session store instead of creating a different store (#17507)

* Simplify Gothic to use our session store instead of creating a different store

We have been using xormstore to provide a separate session store for our OAuth2 logins
however, this relies on using gorilla context and some doubling of our session storing.
We can however, simplify and simply use our own chi-based session store. Thus removing
a cookie and some of the weirdness with missing contexts.

Signed-off-by: Andrew Thornton <art27@cantab.net>
* as per review

Signed-off-by: Andrew Thornton <art27@cantab.net>
* as per review

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Handle MaxTokenLength

Signed-off-by: Andrew Thornton <art27@cantab.net>
* oops

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lauris BH <lauris@nix.lv>