add task cancellations in the UI

[DaveDarsa]

Ability to cancel tasks in the ui tasks page and individual task page

closes: #137

[shreddedbacon]

I like how this adds the cancellation button to the overview list for tasks.

Would it be possible to also do this on the deployments list?

The > in the table row probably doesn't need to exist in either tables too, it seems irrelevant

[DaveDarsa]

Sure thing 👍

[tobybellwood]

Looks good, and works as expected - two minor nits on the error display when trying to cancel without permissions:

[DaveDarsa]

@tobybellwood I think instead of cramming the error in the table, there should be an error notification like so

[tobybellwood]

There's also the cancel deployment error that inlines (even worse than tasks) @DaveDarsa ?

[DaveDarsa]

@tobybellwood should be good to go

[shreddedbacon]

Does this error popup change all errors to this? Or just tasks/deployments?

[DaveDarsa]

@shreddedbacon just tasks/task/deployments/deployment cancel buttons