Support copy from right in timetable editor

[miles-grant-ibigroup]

Checklist

• Appropriate branch selected (all PRs must first be merged to dev before they can be merged to master)
• Any modified or new methods or classes have helpful JSDoc and code is thoroughly commented
• The description lists all applicable issues this PR seeks to resolve
• The description lists any configuration setting(s) that differ from the default settings
• All tests and CI builds passing

Description

Timetable editor has a cool keyboard shortcut to copy from left/top, but nothing to copy from right! We add shift+l to do this, mirroring shift+'.

Please test thoroughly I could only come up with so many edge cases. Tempted to add a new e2e case

[binh-dam-ibigroup]

Add null checks and a check to avoid copying time into a non-time cell.

[binh-dam-ibigroup]

Thanks for the changes