Use v-safe-html for field help texts in integration form

What does this MR do?

This isn't critical since the help texts are defined in the integration models (in Integration#help and Integration#fields) and can be trusted.

We still want to allow external links using target="blank", so we pass a custom configuration to v-safe-html until this is implemented in gitlab-ui with gitlab-ui#1427 (closed).

We also do the same for the main help texts in integration_form.vue, and add the target and rel attributes where missing.

Thanks to @justin_ho who provided the initial patch for this! 🙇

Issue: #241874 (closed)

Screenshots or Screencasts (strongly suggested)

This change should not have any visible impact. Some examples from integrations with HTML in their help texts:

Campfire:

Unify Circuit:

How to setup and validate locally (strongly suggested)

1. Create a project
2. Navigate to Settings → Integrations
3. Click one of the integrations
4. Verify that all HTML renders correctly in the gray box at the top, and in the help labels below each field.

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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.

Related to #241874 (closed)