[Feature] Use Iconify to let developers use whatever icon font they want

[tabacitu]

Currently, we use Line-Awesome as our default icon font. Line-Awesome is great, it's definitely more modern than the standard-width Font-Awesome, and it has the same syntax as it so it's got zero learning curve for most Bootstrap developers.

But. What if we used Iconify? This should allow:

• us to use a font collection we recommend, by default;
• developers to use whatever they want, if they don't like the collection we picked, or can't find an icon in the default collection;

I think that would be a very cool feature. One syntax to rule them all.

To investigate:

• can this work offline, without loading stuff from CDNs?
• would the page be faster or slower to load?

Problems:

• Iconify does NOT support Line-Awesome, so the default icon set needs to change too; Maybe Feather icons?