Rename getSlug to slug

another 2 % of #2124
- renamed `/ghost/api/v0.1/posts/getSlug/ to
`/ghost/api/v0.1/posts/slug/`
- renamed method getSlug to generateSlug
Add functional tests for admin navigation bar and menus

issue #2273
- Check that menu nav items exist
- Check that menu nav items are labeled correctly
- Check that menu nav items have correct URLs
Deduplicate upper-/lowercase tags for posts

server side check for #2478
- added check for duplicate tags in post model
Ensure generateSlug gets transaction for tags

fixes #2460

- add transacting to tag saving
- add same to users while we're here
Merge pull request #2502 from ErisDS/issue-2460

Ensure generateSlug gets transaction for tags
Merge pull request #2498 from sebgie/duplicate-tags

Deduplicate upper-/lowercase tags for posts
Bump version to 0.4.2-rc2
Error handling for require-tree

fixes #2507

- ideally we would create an apps directory if one isn't present, but we can deal with that in later versions
Merge pull request #2508 from ErisDS/issue-2507

Error handling for require-tree
Cache invalidation for tag pages
Updating Ghost-UI to 0.1.3
Version bump for 0.4.2
Fixing showdown link to work without git
Added populate last login time

closes #2515
- sets last_login when user is saved if user status changed to active
- added integration test in user Basic Operations
Fixes tag cache test issues introduced in c6c4d5a8
Merge pull request #2519 from halfdan/tag-cache

Fixes tag cache test issues introduced in c6c4d5a8
Merge pull request #2439 from sebgie/move-slug

Rename getSlug to slug
Fixed check for a theme's custom error.hbs:

Closes #2513
- Checks for property `error.hbs` on active theme
- Added unit test to ensure `error` view is rendered when activeTheme has
  a custom error template.
- Removed unused variable, `userErrorTemplatePath` from errorHandler
- Refactored errorHandler.`updateActiveTheme` to take one argument, the new active theme, and to then check if the active theme has an error.hbs
- Changed errorHandler unit test to use rewire for mocking config.
Adding database to bug report template
Update README & Contributing with details of stable branch