Restyled language selector

- flags replaced with SVG speech bubble + language code
- locale switcher styled as joined button group
Styling for dashboard language picker
Specs passing

- added ids to links in #swtch_locale_picker
- modified feature spec to click on links by id, not text
Replaces flags on pages index
Specs passing
CSS tweaks

- tightened line spacing in locale icon
- lined up text in locale button on dashboard
- bottom margin in locale switcher on page edit form, so links a separated if the switcher wraps to a second line
Merge pull request #2723 from isaacfreeman/2722_locale_selectors

2722 locale selectors

Closes #2722