Make URLs in confirmation panels clickable, and underline them

[stefanhaller]

• PR Description

This is especially helpful for the breaking changes popup, which has a link to the release notes, but it could also be useful for other panels that display some warning or error with a link to more information.

• Please check if the PR fulfills these requirements

• [x] Cheatsheets are up-to-date (run go generate ./...)
• [x] Code has been formatted (see here)
• [x] Tests have been added/updated (see here for the integration test guide)
• [ ] Text is internationalised (see here)
• [ ] Docs (specifically docs/Config.md) have been updated if necessary
• [x] You've read through your own file changes for silly mistakes etc

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: -0.04% (target: -2.00%)	:white_check_mark: 54.90%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (1cedfa463b83b1d50fdba5abec5d16d21ffd36a8)	50365	42372	84.13%
Head commit (5d509efe19b4f17b971470dadca6e1d18ea3c369)	50405 (+40)	42388 (+16)	84.09% (-0.04%)

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3446)	51	28	54.90%

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[jesseduffield]

Thoughts on using tcell's terminal hyperlinks ability? https://github.com/gdamore/tcell/issues/300

[stefanhaller]

I had no idea this existed. I'll play with this to see if it's useful for our case.

[stefanhaller]

So I'm not very thrilled by tcell's hyperlinks. I couldn't get it to work in Terminal.app, only in iTerm2 (but I may well have done something wrong). In general it's a bit unclear to me how widely supported the feature is yet.

Also, you have to hold down Command while clicking them (ctrl on Windows), which I find inconvenient, and hard to discover. I would prefer them to behave more like links on a web page; just click them.

Finally, it would take some effort to make this available in gocui, it's actually not quite clear to me how to design the APIs. There's plenty of code that just passes around fg and bg Attributes to functions (and those are enums); but we need to pass attached data for this one. It would take a while to add this to the current design of how attributes work in gocui.

To sum it up, I'd prefer to stick with the more pedestrian approach I'm taking here.