refactor!: move go-chi/session into Gitea (#39504)

The `gitea.com/go-chi/session` package only exists for Gitea, so it
moves into `modules/session` to fix its bugs directly. Fixes the flake
in
https://github.com/go-gitea/gitea/actions/runs/36726154500/job/109923538400.

- Sessions are only written back when changed, so a read-only request
can't revert a concurrent change or restore a logged-out session, like
https://github.com/go-macaron/session/commit/ae808a4a4660c802965c834299ab08f167effd12
- The session cookie is only set once a session holds data
- Every backend refreshes the expiry on load and file sessions are
written atomically
- Also fix  https://github.com/go-gitea/gitea/issues/36176

## ⚠️ BREAKING ⚠️

* the `mysql`, `postgres`, `couchbase` and `memcache` session providers
are removed, use `file`, `db` or `redis` instead
* login-related cookies are renamed to `gitea_session` and
`gitea_remember`, if you'd like to use the old names, set `COOKIE_NAME`
and `COOKIE_REMEMBER_NAME` in app.ini

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/util/util.go

@@ -118,6 +118,16 @@ func FastCryptoRandomBytes(length int) []byte {
 	return buf
 }
 
+func FastCryptoRandomString(length int, chars string) string {
+	buf := make([]byte, length)
+	limit := int64(len(chars))
+	for i := range buf {
+		num := FastCryptoRandomInt(limit)
+		buf[i] = chars[num]
+	}
+	return string(buf)
+}
+
 func FastCryptoRandomHex(length int) string {
 	buf := FastCryptoRandomBytes(length / 2)
 	return hex.EncodeToString(buf)