Add index to AccessToken.token


Hello,

this morning I found a performance problem related to the AccessToken table. With around 3k tokens, the get_token query takes around 1 ms to run, but with 30k, takes more than 20ms.

I added the index and the migration for the token field. For this specific case, I don't think tests are relevant .
And last, I didn't bump the version, so do it if you accept the PR.

Thanks