Move existing captcha logic to plugin (fixes #6148)

[Nutomic]

The captcha API works exactly like before, only that the logic is in a wasm plugin now. However it needs a change in lemmy-ui to load the captcha unconditionally, as local_site.captcha_enabled is gone.

Plugin PR: LemmyNet/lemmy-plugins#9

TODO:

• Make captcha validation work
• Remove all db tables and local_site fields related to captcha
• Regenerate and reenable patch file for diesel schema
• Possibly change config to allow a single captcha plugin, instead of looping through all plugins
• Remove changes to release profile

[Nutomic]

Once this is merged we should tag a new alpha version, and use that in lemmy-plugins repo instead of committing the binary directly to the repo. Then we can clear the history to reduce repo size as described in LemmyNet/lemmy-statistics#3.