log: load file incrementally

[oliver-sanders]

Will pick up this PR sometime after #2017 (merged)
Will pick up this PR sometime after #2106 (merged)

• Add log lines using events rather than Vue reactivity.
• Should reduce the time taken to add a new log line from O(N) to O(1).
• This should allow us more flexibility in choosing the batch size used to send log lines to the UI for faster large-file load in the future.
• We were storing the log lines in a JS array AND in the DOM elements. This removes the JS array so should reduce the RAM overheads too.
• Handle show/hide timestamps with CSS rather than JS (also allows for styling timestamps)

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.