Feat/custom icon upload

[BrutalBirdie]

This adds the option to upload a custom icon and would fix #145.

Screenshot of the edit page:

Screenshot of the login page:

[julianlam]

Hi @BrutalBirdie, a couple concerns:

1. I'm not entirely sure whether a re-implementation of upload logic is necessary. Have you look into using the methods in src/controllers/upload.js? I realize that the logic there is rather specific to NodeBB and there are hard-coded bits, but using those (and modifying them for plugin use-cases) is better worth the time than writing your own upload logic.
2. Injecting your own CSS via the new route (+ client.js) is not necessary. The base styles can be added to a new SCSS file, and instead of a custom class, just set icons.svg in the returned strategy object. It's expecting an SVG element, but you can probably just pass in your own <i> element and set an inline-style for the background-image.

That should simplify things a bit.

[BrutalBirdie]

Hello @julianlam
Thanks for the much-needed feedback.
As you might have noticed, I just wrote code that I knew would work 😅
To be 100% transparent, I have no idea how to develop a nodebb plugin.
So this PR can be viewed more like a "proof of concept".

If you, or someone, could take this PR and rewrite it into the best version suitable for nodebb, that would be awesome, and I can use that difference to learn how it should be done. :)