web server instead of batch report generation

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Addresses #14

TODOs:

• separate server binary (easier that way, offline/online could be united to single binary later)
• 3-legged Oauth2.0 flow support
• multitenancy: store session tokens per-user (in cookies)
• multitenancy: allow to choose a label name, store it per-user
• aggregation & HTML rendering of messages
• better abstract split (unicode friendly, whitespace-aware)
• automatic token refresh automation will be added later,
  it's enough that it redirects to /login on token expiration now
• add API fetch progress indicator
• main page rendering using html/template

[bzz]

Current implementation is working 🎉
it just does include the report template yet.

It's a tad slow to be interactive, as one need to fetch every every message in individual API call (until googleapis/google-api-go-client#435), so added another item about visual progress indicator for a better user feedback.

[bzz]

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Initial version of the server is working, going to polish just a few be bits and merge it asap as it includes useful refactoring/improvements to the CLI as well.

Further usability improvements (message loading time, better template rendering, etc) will be handled under a separate issues.

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Rebased on latest master & ready to