Use BEM for Styles

# Problem

We want to standardize our stylesheets using [BEM](http://getbem.com/introduction/). This is a master issue to track all needed refactoring.

The lists below correspond to stylesheets. Each stylesheet (except those with `_` in front of the name) correspond to a `.hbs` template file. General styles will need to be renamed to fit the naming conventions, which then need to be changed everywhere they exist in templates. Take one `_name.scss` file at a time. Create a separate issue and reference this master issue. Take advantage of `git grep` to find all references to that style. If you are making changes to styles within components or templates, feel free to create a grouped issue and pull request (as in #673) Once each issue and it's related PR's are complete, check off the files refactored on this list. Make sure you make changes in both the `.scss` and `.hbs`!

# Files to Refactor
## General
- [x] _alerts.scss
- [x] _animations.scss
- [ ] _buttons.scss
- [x] _code.scss
- [x] _colors.scss
- [x] _fonts.scss
- [ ] _forms.scss
- [x] _header.scss
- [x] _icons.scss
- [ ] _images.scss
- [ ] _inputs.scss
- [x] _misc.scss
- [x] _pace.scss
- [x] _retina.scss
- [x] _settings.scss

## Templates
- [x] app.scss
- [x] dropdown-menu.scss
- [x] index.scss
- [x] main.scss
- [x] start.scss
- [x] task.scss
- [ ] task.scss
- [ ] task-new.scss
- [ ] templates/about.scss
- [ ] templates/team.scss
- [x] templates/project/index.scss
- [x] templates/project/settings/contributors.scss
- [x] templates/start/expertiese.scss
- [x] templates/start/hello.scss
- [x] templates/start/skills.scss

## Components
- [ ] donations/donation-progress.scss
- [ ] donations/donation-status.scss
- [x] categories-list.scss
- [x] category-item.scss
- [ ] code-theme-selector.scss
- [ ] create-comment-form.scss
- [ ] donation-goal-edit.scss
- [ ] donation-goal.scss
- [ ] donation-payment.scss
- [x] drag-zone.scss
- [ ] editor-with-preview.scss
- [ ] error-wrapper.scss
- [x] image-drop.scss
- [x] login-form.scss
- [x] member-list-item.scss
- [x] navigation-menu.scss
- [ ] organization-header.scss
- [ ] organization-members.scss
- [ ] pager-control.scss
- [x] progress-bar.scss
- [ ] project-card-members.scss
- [ ] project-card-skills.scss
- [ ] project-card.scss
- [x] project-details.scss
- [x] project-header.scss
- [ ] project-item.scss
- [x] project-list.scss
- [x] project-long-description.scss
- [x] project-menu.scss
- [ ] project-task-list.scss
- [ ] role-item.scss
- [x] settings-menu.scss
- [x] signup-form.scss
- [x] site-footer.scss
- [ ] stripe-connect-button.scss
- [x] task-comment-list.scss
- [ ] task-details.scss
- [ ] task-header.scss
- [ ] task-item.scss
- [ ] task-new-form.scss
- [ ] task-status-button.scss
- [ ] task-status.scss
- [ ] task-title.scss
- [x] user-details.scss
- [x] user-menu.scss
- [ ] user-organizations-list.scss
- [ ] user-sidebar.scss
- [ ] user-skills-input.scss