Add support for Content-Security-Policy

Closes #65330

See merge request gitlab-org/gitlab-ce!31402

Kubernetes deployments on new clusters will now have
a separate namespace per project environment, instead
of sharing a single namespace for the project.

Behaviour of existing clusters is unchanged.

All new functionality is controlled by the
:kubernetes_namespace_per_environment feature flag,
which is safe to enable/disable at any time.

- Closes #60024

- Change PrometheusClient.new to accept a base url instead of an
  already created RestClient

- Use Gitlab::HTTP in PrometheusClient instead of creating RestClient
  in PrometheusService

- Move http_options from PrometheusService to
  PrometheusClient (follow_redirects: false)

- ensure that base urls don't have the trailing slash

- Created a `PrometheusClient#url` method that might not be strictly
  required

- Change rescued exceptions from RestClient::* to
  HTTParty::ResponseError where possible and StandardError for the
  rest

A nonce-based Content-Security-Policy thwarts XSS attacks by allowing
inline JavaScript to execute if the script nonce matches the header
value. Rails 5.2 supports nonce-based Content-Security-Policy headers,
so provide configuration to enable this and make it work.

To support this, we need to change all `:javascript` HAML filters to the
following form:

```
= javascript_tag nonce: true do
  :plain
    ...
```

We use `%script` throughout our HAML to store JSON and other text, but
since this doesn't execute, browsers don't appear to block this content
from being used and require the nonce value to be present.

Support selective highlighting of lines

See merge request gitlab-org/gitlab-ce!31361

Make issue boards importable

See merge request gitlab-org/gitlab-ce!31434

Instead of highlighting all lines when not all of them are
needed, only highlight specific lines.

The `BlobPresenter#highlight` method has been updated to
support `since` and `to` params. These params will be used to
limit the content to be highlighted.

Modify `Gitlab::Highlight` to support `since` param which will
then be used to determine the starting line number.

Add project path to sql query to build proper path

API endpoints for requesting container repositories
and container repositories with their tag information
are enabled for users that want to specify the group
containing the repository rather than the specific project.