Ability to login to multiuser using javascript

**Is your feature request related to a problem? Please describe.**
currently there's no elegant way to login to multiuser plugin using only javascript

**Describe the solution you'd like**
i want a easy and concise way to login to X masterseed autonomously with 0 user input and only using javascript
ideally also having a new site perm like "multiuser manage"

**Describe alternatives you've considered**
the farthest possible is to prompt user for the masterseed input
the only way right now is to also get a userscript to act on your behalf filling in the masterseed form
which can get messy trying to communicate out of iframe and back into iframes

Read the multiuser plugin's existing login flow and the iframe communication described in the issue, then identify the JavaScript entry points and permission checks involved. Done should mean a concise autonomous login path for the X masterseed with no user input and a defined "multiuser manage" site permission, without relying on a userscript to bridge iframes.

索引モデルが issue の本文から書いたものです。