Add support for export/import to Gitlab snippets

Feature Request

What would you like to be able to do?

Export and import config to Gitlab in the same manner as Github.

Support both gitlab.com and internal gitlab instances.

Snippets API: https://docs.gitlab.com/ee/api/project_snippets.html

API Examples:

http POST https://gitlab.com/api/v4/snippets?private_token=$PRIVATE_TOKEN \
    title="Nevegreen Config Backup" \
    file_name=nevergreen.json \
    visibility=public \
    content=@nevergreen-config.json

http GET https://gitlab.com/api/v4/snippets/$SNIPPET_ID/raw?private_token=$PRIVATE_TOKEN 

http PUT https://gitlab.com/api/v4/snippets/$SNIPPET_ID?private_token=$PRIVATE_TOKEN \
    title="Nevegreen Config Backup" \
    file_name=nevergreen.json \
    content=@nevergreen-config.json

Remaining Tasks

Prod code:

• Handle error messages from GitLab
• Update schema to add new GitLab config
• Allow custom hosts to be added to the CSP (similar to iframe)
• Help text
• Mobile styling (issue with tabs overflowing)

[GentlemanHal]

FYI, if we do it from the UI we’ll need to update the content security policy connect-src to include gitlab.

[joejag]

Tested with a hosted GitLab. All working.

We could improve things by pre-populating the available URLs given the environment variable provided by the user, but given [#271] will make the environment variable redudant I’d suggest we don’t do this just now.