Add Vue.js-based button for extending/reactivating trial

What does this MR do?

This MR adds a new Vue.js based button for extending or reactivating the trial. The button is not active yet the 3rd MR will enable it but it's still hidden behind the feature flag allow_extend_reactivate_trial.

This is the 2nd merge request out of 3:

1. Add helper methods
2. Add Vue.js button (this one)
3. Add initialization code and feature tests

See #321577 (closed) for more information.

Screenshots or Screencasts (strongly suggested)

Extending trial

State	Image
Initial
Button clicked
When trial has been extended

In action

extending_trial-2021-07-13_11.48.57

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.

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