Convert http/servlets/registerservlet.py to use async/await

[H-Shay]

This PR converts a function in http/servlets/registerservlet.py to use async/await. A few other files/functions needed to be changed as well.
One note: on line 109 of http/servlets/registerservlet.py, I removed the keyword yield from the call to issueToken because when I substituted await there, a few of the tests failed. After adding a breakpoint, I discovered that the call to renderPost was failing with the following message: twisted.python.failure.Failure builtins.TypeError: object str can't be used in 'await' expression. Mypy had also flagged this as a problem. I checked the function definition of issueToken and as far as I can tell it is not an asynchronous function so I am unclear as to why is was ever yielded to/awaited in the first place. Removing the await resulted in the tests all passing, so perhaps it was the right thing to do?

[clokep]

One note: on line 109 of http/servlets/registerservlet.py, I removed the keyword yield from the call to issueToken because when I substituted await there, a few of the tests failed. After adding a breakpoint, I discovered that the call to renderPost was failing with the following message: twisted.python.failure.Failure builtins.TypeError: object str can't be used in 'await' expression. Mypy had also flagged this as a problem. I checked the function definition of issueToken and as far as I can tell it is not an asynchronous function so I am unclear as to why is was ever yielded to/awaited in the first place. Removing the await resulted in the tests all passing, so perhaps it was the right thing to do?

It's possible it returned a Deferred in the past, but inlineCallbacks let you yield on "dumb" values (I think the idea behind it is to let you switch out implementation that might return a Deferred or something). Anyway, async/await doesn't allow this and the change you made is the correct one. 👍