Changes proposed in this Pull Request

This PR updates the following to use sentence case:

• Card headings
• Form labels

Page titles remain in title case.
All other titles can be in sentence case, except when referring to an official/branded feature or service name.
Taglines, subheadings, descriptions, etc. should always end with a period (and, ideally, be actual sentences rather than fragments).

For more info: p2-pbAPfg-dH

Before (card heading):

After (card heading):

Before (form label):

After (form label):

Before (/plans):

After (/plans):

Testing instructions

• Checkout this branch locally or visit https://calypso.live/?branch=card-headings-form-labels-sentence-case
• Go through each section looking and make sure card headings and form labels are written in sentence case
• Try with different site types (Simple, Atomic, Jetpack), different plans (free, paid), or any other combination you can think of
• Please note that some pages in checkout will remain in title case. They will need to be updated in the backend. See: #41768

Sections:

• My Home
• Stats – Fixes: #40622
• Plan
• Import
• Export
• Marketing
• Activity
• Hosting configuration
• Domains
• People
• Settings

/me

• My profile
• Account settings
• Manage purchase
• Security
• Privacy
• Notification settings
• Blocked sites

cc @rickybanister @lcollette @gwwar @kwight