[GitHub Desktop] Disabling repo indicators feature

[janiceilene]

Anyone is welcome to open a PR!

Use this Desktop issue, Support disabling repository indicators, to understand the context of this change. Download the current beta version of Desktop to take any screenshots.

This will require an updated image for both windows and mac, because it's a change in the settings (which have different names in each OS).

Audience: Every Desktop user will come across this updated tab if they go into the settings at all. The new tab is particularly relevant for people who work on underpowered machines or who have a ton of repositories in Desktop and there are performance concerns related to the background operations of fetching to update these indicators.

In Configuring basic settings:

• Update the image in step two in both windows and mac, to include the new "Prompts" tab.
• Add a line to the bulleted list for Prompts, modeled after the other list items.