EmbedSVG icons instead of icon fonts

[rafalp]

Looks like I was mistaken about insisting to keep using icon font in Misago, with "better" practice being embedding SVG directly in the document.

Pros:

• You only send client the icons that are used on the page.
• Gzip is very good at compressing repeated uses of same icons in the document.
• You can include thousands of icons in Misago for plugins and devs to use without increasing download sizes.

The plan

Create a new misago.icons app that will implement IconLoader class that will load icons by name into memory and will return their SVG markup. Icons will be located in misago/icons/tabler directory, but plugins should be able to tell the loader "hey, also look for icons in my own materialdesign directory"

Also implement {% icon "tabler/name" %} template tag that will call this icon loader and return desired icon's SVG or raise an error. {% icon "tabler/name" optional %} shouldn't raise an error if icon could not be found.