fix #215: add GA event for finished sign-in

[groovecoder]

@clarkbw - please check this and verify these are good event category, action, and label values for Google Analytics. I can make them whatever we want, but we should be sure about them now so we can maintain the data integrity after launch.

[clarkbw]

👍 Looks good. Is there any error actions available?

[groovecoder]

We won't know if there are errors happening on the FxA sign-in page. Let me see if I can record if/when a user gets an error from the dashboard after getting back from FxA ...

[groovecoder]

Okay, I added a GA event if/when the user gets a dashboard error page after trying to sign in with Firefox Accounts. The only way I could trigger this was to corrupt my FxA Oauth Client Secret, so I don't expect we'll see this error much at all once everything is set up.

[codecov-io]

Current coverage is 92.38%

Merging #216 into master will not change coverage

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