Support more archive extensions in SendArchive

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/60

See merge request !66

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/60

Pass version as 'release' to Sentry



See merge request !64

Add Sentry remote error collection

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/57

Vendors:
- https://github.com/getsentry/raven-go (unnamed permissive license)
- https://github.com/certifi/gocertifi (Mozilla Public License 2.0)

Discussed licenses with @JobV; they seem OK.

See merge request !63

API safeties

Increase security and sanity checks to allow for new features.

- ability to exchange signed information with gitlab-rails via a shared secret
- content-type checks to prevent leaking internal API data

Signed messages are implemented using JWT. To make this possible we vendor the jwt-go library.

Companion MR in gitlab-rails: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5907

See merge request !60

Fix backend URL parsing

Our fancy custom flag parser broke in Go 1.7. Also, the new 'strict
TCP connection' feature was not handling backend hosts like
`localhost:3000` (it was panicking).

Fixes:

https://gitlab.com/gitlab-org/gitlab-workhorse/issues/54
https://gitlab.com/gitlab-org/gitlab-workhorse/issues/53

See merge request !61

From https://github.com/dgrijalva/jwt-go . This library is MIT
licensed just like gitlab-workhorse itself.

Fix typo in header name



See merge request !59