Configure backgrounds for add note view

[ctarda]

Fixes #1340

The scope of the PR is limited to just making the screen to Add a Note usable in Dark mode when building the codebase with the release version of Xcode 11.

Before:

After:

Note that the cell view marked with an arrow is fixed in PR #1337

Changes

• Update the background of TextViewTableViewCell
• Override dark mode for the text view embedded in it. It seemed to be a simpler solution, and simpler to revert, that preparing its background and font colors to support dark mode later on.

Review

• Set a device in dark mode
• Build the branch, navigate to an order in Processing status, and Add a Note.
• If PR Set background colour of cells used in Settings #1337 has already been merged, the view should look ok. If it hasn't, the view should only look fine, except for the section corresponding to "Email note to customer:

Update release notes:

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[jaclync]

Testing: ✅ (same behavior as in screenshots)
Code: ✅