registry: make it easier to add pre-module modules

[ovesh]

Right now, the /mods/find page will list repo tags and a pseudo-version of HEAD per user-input line (for github repos only).

• the pseudo-version syntax doesn't follow the standard
• Users have do some copy-pasting into /mods/new

1. fix the pseudo-version
2. add UI that will allow to easily register any of the listed version.

[ovesh]

this is what the UI flow looks like in my branch:

/mods/find

/mods/new

Note that the URL in /mods/new is a GET form. I figured that this is preferred because the request doesn't change state, but I'm ok with changing it.