Redesign lecture edit page for lecturers

[Splines]

Current preview:

TODO

• [x] Add deep linking according to this guide and this SO answer
• [x] Avoid redirect with Cancel button
• [x] Fix layout issues
• [x] Fix sample DB Save & Cancel buttons
• [x] Fix Comments save button not saving any changes (it even triggers an exception)

For reviewers

Sorry for the over 1,000 line insertions/deletions in this PR. This is mainly due to having migrated the lectures.coffee file to lectures.js and due to copying/pasting some old content to new locations while also modifying it, such that Git does not recognize it as replacements and instead deletions/insertions.

• [ ] Headers should be internationalized
• [ ] Every functionality that was in the accordion beforehand, should still work.
• [ ] Clicking on the "cancel" button should not redirect to the "content" tab but stay in the current tab
• [ ] Jumping directly to a tab via a url should work, e.g. this one
• [ ] Accessibility: navbar header can be navigated by using Tab, then Arrow keys (up down as well as left right)
• [ ] URL hashes (e.g. "#settings") should be a good naming choice as they shouldn't be changed in the future very often (to allow easy bookmarking for example)
• [ ] Announcements should be scrollable (reproduce by creating at least 10 new announcements)
• [ ] Importing media should still work.