Add UI feedback after upload + allow user to retry upload

[robertodauria]

This PR makes quite a few changes to the extension:

• The Upload feature is now configurable via a new section in the Settings
• If the upload is enabled, it will attempt to upload the new measurement at the end of a test, displaying the result to the user
• If the upload has failed, it's possible to retry uploading old measurements from the "Explore Data" section, by opening the contextual menu on each entry (swiping to the left) and clicking the upload icon.

[critzo]

LGTM @robertodauria
One question about the ability to attempt upload again if it initially fails. I see you're enabling a manual upload button on results that weren't uploaded originally. That's cool. Is it possible for the upload service to automatically upload previously un-uploadable results? Maybe the next time it tries uploading a new result? If not, that's ok, I just wonder about automating that instead of just relying on a user to take action.

[robertodauria]

Discussed on Slack. The short summary is: it can be done, but we need either to preserve compatibility with older measurements or exclude them from this feature somehow.