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

Start with one unchecked stylesheet from the issue, such as _buttons.scss, _forms.scss, or a listed component stylesheet, and use git grep to find its references in the corresponding .hbs template. Review the BEM naming guidance linked in the issue, then create a separate issue for that stylesheet. Done means the SCSS and all referenced templates use the standardized names and the master checklist is updated.

Written by the indexing model from the issue text.