- `no_custom` -> `no_themes` (history: before themes existed, we had a similar tool called 'customizations')
- `only_official` -> `no_unofficial_plugins` (matches format of `no_themes` and `no_plugins`, and makes it clear that this doesn't affect themes)
- `?safe_mode=no_themes%2C%no_plugins` -> `?safe_mode=no_themes,no_plugins` (the query portion of a URL does not require commas to be encoded. This is much nicer to read)
- If `no_plugins` is chosen from `/safe-mode` the URL generated will omit the superfluous `no_unofficial_plugins` flag
- Some tweaks to copy on `/safe-mode`
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
