Reroute CI Minutes purchase for personal namespace

What does this MR do?

Related issue: #338724 (closed)

Reroute CI Minutes purchase for personal namespace to CustomersDot flow

Screenshots or Screencasts (strongly suggested)

For a personal namespace

Screen_Recording_2021-08-19_at_12.39.17

For a group namespace

Screen_Recording_2021-08-19_at_12.38.49

How to setup and validate locally (strongly suggested)

1. Enable the feature

   Feature.enable(:new_route_ci_minutes_purchase)

Expected behavior for a group namespace (didn't change)

1. Open a group that is eligible for CI Minutes purchase
2. Choose Usage quotas in the left menu -> Buy additional minutes -> You should be redirected to -/subscriptions/buy_minutes?selected_group=<this_group_id>

Expected behavior for a personal namespace (changed)

1. Open your personal Preferences
2. Choose Usage quotas in the left menu -> Buy additional minutes -> You should be redirected to CustomersDot /buy_pipeline_minutes

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.