Add helper methods for extending and reactivating trial

What does this MR do?

This MR is about adding two helper methods that will be used in next merge requests for displaying the button for extending or reactivating the trial:

1. show_extend_reactivate_trial_button? - for checking if we need to display the button
2. extend_reactivate_trial_button_data - for returning a Hash with initial data for this Vue.js-based button.

This is the 1st merge request out of 3:

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

See #321577 (closed) for more information.

Screenshots or Screencasts (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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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)
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• This change is backwards compatible across updates, or this does not apply.

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• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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