Price override support enhancements

[pierre]

In #139, we added support for overriding the price of the first phase of the plan. We should enhance the UI to be able to specify overrides per phase (e.g. to override the evergreen price only, or both the trial and the evergreen prices).

[reshmabidikar]

I'm thinking we can have UI similar to the following:

1. Phase overrides section should be collapsible
2. When expanded, it should show a row populated with the first phase in the plan
3. In case there are additional phases, the "+ Add Another Phase" link should be shown
4. On clicking this link, another row should be created. It should be populated with the remaining phases from the plan

Thoughts @pierre?

[pierre]

I like it! 👍 Also, the phases dropdown should be populated from the catalog (to match the plan phases).

[tungleduyxyz]

Update UI:

[reshmabidikar]

Tested with 4.0.21-593e019 and it looks good overall 👏

Suggesting a few improvements:

1. Change the Phase Overrides label to Price Override
2. By default, keep this section collapsed
3. Don't show the "Add another phase" button when overridden price rows are created for all the phases in the plan. So if there are two phases in the plan and the user has already created price overrides for both the phases, no need to show the "Add another phase" button, this does not make sense.