Clean individual tables

I'd ask someone to test this (probably someone not currently in Iceland). The pattern for testing is as follows (buttons to do this are all on the database action screen and labeled appropriately):

• Gather both reading and streaming data.
• Test clearing the readings.
• Test clearing the streaming.
• Gather both reading and streaming data.
• Test clearing all field data.

It's probably worth doing an uninstall and a reinstall of the app for this.

The last thing I'd note is this makes the database screen really cramped on small devices like phones. Methinks actions and details should be split into separate screens in a later version.

(I erroneously named the branch "clean-individual-databases" rather than "clean-individual-tables" but in reality it cleans tables. 🙃 )