Android below 7.1 no longer supported (#6210)

In reality, support for Android versions below 7.1 has already been just on-paper for a while now, as more and more issues with old Androids have been popping up lately that cannot be fixed on our side. Luckily, it seems that >99.6% of app users already use Android 7.1 and above. (See #6210)

Main screen improvements (#6213)

Cleaned up the appearance of the main screen a bit:

• Merged the +/- buttons into one button, not shown by default, it's a setting. By the way, you can also zoom with one finger by double-tapping, then moving the finger up or down. This works in any app in which you can zoom, actually.
• The upload button is now in the main menu
• The pointer pin button moves more smoothly (in an ellipsis near the screen border)
• Added a scale bar on the lower right (#5765)
• Replaced the static map attribution text with an info button

New overlays

• MTB difficulty: Add the difficulty ratings for mountainbike trails (#1850, #5726), thanks @RubenKelevra and @ravenfield. It is disabled by default because you need to be a domain expert to accurately record this data. (#6226)

New quests

• "Are gluten-free products offered here?", disabled by default (#3031, #6143), by @mnalis

Quest and Overlay improvements

• Opening hours: Fix semantic issue when specifying off-days at the top (#6191)
• Surfaces: Fix surface was asked twice when a generic surface was selected (#6172)
• Smoothness: Don't ask if surface is still correct just after specifying the surface smoothness (#6178)
• Baby changing tables: Ask at more places (#6167), by @agent-redd
• Sidewalk overlay: Align which roads are selectable with the bike paths overlay, for consistency (#6185), by @mnalis
• Roadwork: Also ask whether minor roadwork (i.e. construction without full road closure) is completed (#6090)
• Various: Ask about more features of alpine and wilderness huts (#6200, #6201), thanks @tiptoptom
• Building levels: Prettified input form a bit (#6022), by @GaeaKat
• Other small things (#6209, #6216, #6217…), thanks @mnalis

General Improvements

• Performance improvements on upload and download (#6150) as well as significantly decreased data download size (#6218)
• Performance improvements in the quest selection screen (#6205), by @irakliy01
• Added a few new links to unlock via achievements. Newly added links do not unlock silently for old users anymore (#6220)
• Render parks and playgrounds with an outline (#6221), by @doniks