Instrument welcome screen events (#3930)

# TODO

- [x] Fix busted tests
- [x] Check all events from actions on welcome screen
    - [x] change theme
    - [x] change keymap
    - [x] install cli
    - [x] enable vim
    - [x] send metric data
    - [x] send diagnostics
    - [x] close welcome screen
- [X] Check all settings events
    - [x] record theme change (in main and in selector)
    - [x] record base keymap change (in main and in selector)
    - [x] record metric data value
    - [x] record diagnostics value
- [X] Triple check that telemetry settings still block events from being
sent, since we updated how we check for settings changes
- [X] Remove immediate flush mode and allow the flush method to be
called publicly
- [x] Remove references to clickhouse in the code (should be generic at
the client level)

Release Notes:

- N/A