Add linear integration

[HartS]

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Add an integration with Linear (project management / issue tracking tool). This adds a toggl track button in the board view only, and inserts the issue title, project name (as project), and issue ID + labels (as tags), when the button is clicked.

Right now they don't have semantic markup which results in some pretty nasty selectors.

Debugging showed the expected tags were getting sent to the togglbutton.createTimerLink options, but the "Toggl tags" aren't getting selected in the time entry that gets started (even if those tags have already been created). Perhaps this is an issue with the Toggl extension itself?

I asked the Linear team to add some semantic classes, data-* attributes, or even aria- roles that could be used to construct less brittle selectors, so I'll update this PR (or submit a new one) to improve things if they're willing to oblige.