Charlie/add sustainer icon

[charliescheer]

Fix

As a thank you to our sustainers we are offering them the option to set their app icon to an exclusive icon color. This PR adds that functionality. NOTE: this will only work with existing sustainers, and becoming a sustainer is removed in another PR. In a future PR will figure out how sustainers will continue having access to this option after sustainer is retired.

Settings	Icon

Test

1. Clean install of Simplenote. Log into a non sustainer account. Go to Tags > Settings and confirm that there is no row for enabling the sustainer app icon
2. Go to SPSettingsViewController.swift. and add return true` to L.44 so that you appear to be a sustainer
3. relaunch the app, return to the settings view and confirm that you have a switch under the Sustainer Thank you section
4. Enable the switch. Then exit the app and confirm the icon is now the pretty green color
5. Relaunch the app, return to the settings and confirm that the sustainer app icon is enabled.

Review

(Required) Add instructions for reviewers. For example:

Only one developer and one designer are required to review these changes, but anyone can perform the review.

Release

(Required) Add a concise statement to RELEASE-NOTES.txt if the changes should be included in release notes. Include details about updating the notes in this section. For example:

RELEASE-NOTES.txt was updated in d3adb3ef with:

Added markdown support

If the changes should not be included in release notes, add a statement to this section. For example:

These changes do not require release notes.

[wpmobilebot]

You can test the changes in simplenote-ios from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr1556-b067188-018e80d5-332f-4393-83da-493571c4fddf on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[dangermattic]

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[jleandroperez]

Added a couple notes, code wise looks great!!

when ready!!