[META] Collect Thunder related core issues

This is a meta issue to collect Thunder relevant issues

Content creation:

• #1368140: Improve user experience of the taxonomy/term/X/edit page
• #2596207: More specific titles for term and vocabulary edit forms
• #1343822: Autocreated taxonomy terms duplicated if they are added to multiple fields
• #1899360: Add save dropbutton next to 'Back to content editing' link in node preview.

Media:

• #2492171: Provide options to sanitize filenames (transliterate, lowercase, replace whitespace, etc)

Help:

• #2592487: [meta] Help system overhaul
• #2820166: Flexible plugin based help system

Outside-in:

• #2784443: Move off-canvas functionality from Settings tray module into drupal.dialog.ajax library so that other modules can use it

Demo:

• #2759849: Create a new user-facing core theme as part of a Drupal demo
• #2851086: Tour tip display under toolbar
• #2851270: Links in the tour tip body are visually the same as the rest of the text
• #2409861: Explore replacing obsolete Joyride asset with latest version

System:

• #2776605: Make it possible to specify a destination after the installation for distributions
• #1356276: Allow profiles to define a base/parent profile
• #2186949: Allow a distribution to ship with multiple installation profiles (of which one can be selected in the installer)