Enable remaining bootstrap components

[gravitystorm]

In #2452 I took the approach of starting to use bootstrap with most of the components disabled, then working through them individually to enable them and refactor when necessary. I think it's helpful to have an issue open here to share my plans with everyone who might be interested.

#2508 contained enabling work in order to enable forms, and to remove the bulk of our custom reset code that was interfering with some components. As far as I'm aware, there's only two things now remaining to refactor before enabling bootstrap globally:

• Tooltips. We have our own copy of an older version of bootstrap-tooltips and enabling the built-in tooltips gives white text on a white background
• Dropdowns. We have our own dropdown boxes with similar enough css class names that we end up with two dropdown arrows on each box

When those are taken care of, then we can enable bootstrap entirely and remove our temporary bootstrap-custom.css file (and close this issue). Future work after that point would include:

• Refactoring the forms. There's actually quite a few of these around the site, and as well as making them look better (see e.g. Use bootstrap across site gravitystorm/openstreetmap-website#37) I want to bring some consistency around inline errors vs error blocks. But this is a lot of work.
• Continue to slim down our custom.css file. Using the bootstrap grid has removed a bunch of code. So we're down from 2910 lines to 2609 already, forms will reduce that further, and I hope to find more things where we can just use the framework instead of having to have custom rules.