Add reusable badge for plan level label

In #227 we discussed whether the "Pro plan" label/badge should stick around after a user upgraded. There is some value in keeping the plan name here as it is probably a reminder of the features that come with the plan. However, I usually see services discarding upgrade buttons and labels on features like this after you upgrade. There is also value here as we shouldn't confuse the user by implying they don't have the Pro plan.

• #227
• https://github.com/readthedocs/ext-theme/pull/277/files#r1499465546

We have a need for this in a few areas, it would be nice to have a solution that:

• Takes into account the current plan level and becomes visually distinct when the plan level is already met/exceeded. Most likely we'll want to play around with some visual styles like adding a check mark and making the text muted colors.
• This should be reusable -- either on the template side as an include or on the JS side as a web component maybe. All the information is available to the templates however.
• The label should trigger a popupcard with more detailed information on the plan and current plan level. This would be a template implementation mostly.

Start by reading issue #227 and the linked discussion in pull request #277, then locate the existing plan labels and popupcard templates in ext-theme. The reusable label should reflect the current plan level, look distinct when the level is met or exceeded, and open a popupcard with plan details.