Login button issue in light theme

[skip ci]

add $__unixEpochGroup and $__unixEpochGroupAlias to sql datasources

Doc - fix broken link

removed inverse btn styling and added bgColor to generic oauth and grafana.com login buttons, added styling so log in button uses dark theme inverse btn styling both for dark and light theme

There's no per-row contextual menu anymore
Instead there's a  option

Closes #12862

Closes #10784

Set User-Agent header in datasource proxied requests

Header value will be Grafana/%version%, i.e. Grafana/5.3.0

Fix typo

Fixes #12524

[skip ci]

GitLab authentication backend

If `allowed_groups` is not used with GitLab authentication, the
*read_user* scope can be used instead of *api*.

Set `BasicUserInfo.Id` in the value returned by
`SocialGitlab.UserInfo()`, in case the email address of the user changes
in GitLab. That way, the user association won't be lost in Grafana.

GitLab could already be used as an authentication backend by properly
configuring `auth.generic_oauth`, but then there was no way to authorize
users based on their GitLab group membership.

This commit adds a `auth.gitlab` backend, similar to `auth.github`, with
an `allowed_groups` option that can be set to a list of groups whose
members should be allowed access to Grafana.

Closes #12370

[skip ci]

Karma to Jest: rename and cleanup

[skip ci]

[postgres] add timescaledb option to postgres datasource

WIP Karma to Jest: heatmap renderer (refactor)

Karma to Jest: share_modal_ctrl