Modify CSP when Sentry is configured

What does this MR do?

Fixes #337972 (closed)

This MR makes it so that the CSP adapts automatically when Sentry is configured in gitlab.yml.

At the moment the CSP is enabled only in dev, but in its current state configuring sentry would lead to CSP errors and would block reporting.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Configure sentry settings (doesn't need to be valid sentry DSN for testing this)

     ## Error Reporting and Logging with Sentry
     sentry:
       enabled: true
       # dsn: https://<key>@sentry.io/<project>
       clientside_dsn: https://sentrykey@sentry.example.com/project
       environment: 'development' # e.g. development, staging, production

2. Observe the clientside_dsn (with the key@ part) added to the CSP's connect-src directive in the dev tools or with curl

   $ curl -is https://gitlab.local:3443/ | grep -Eo 'connect-src [^;]*;'
   connect-src 'self' http://gitlab.local:3808 ws://gitlab.local:3808 https://sentry.example.com/project;

   (The gitlab.local:3808 part is the webpack dev server and is unrelated to this change)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team