Add Siri support for overrides

[novalegra]

Currently, overrides aren't visible to Siri (meaning they can't be set with Shortcuts or via voice). There also isn't a great way to view the override history in Loop; the issue report does contain the history, but it's not in a very human-readable format.

This PR:

1. adds a Siri extension & the ability to use overrides with Siri/Shortcuts
2. adds an override history screen
3. makes some under-the-hood improvements to override accounting in order to correctly determine the end date of an override

Some notes about the overrides with Siri implementation:

• I used UserDefaults so the extension could communicate with the main Loop app; the main app has a key-value observer that will watch for changes to set the appropriate override. One issue with this is that the KVO doesn't happen when Loop is in the background, so I changed the intent handler to always .continueInApp so the override will be set. Open to other ways to do this if there's something that could set the override in the background
• I chose to use an extension so it'd be easier to expand Loop's Siri capabilities in the future

LoopKit PR: LoopKit/LoopKit#343
NightscoutService PR: ps2/NightscoutService#4

[novalegra]

I created IntentExtensionInfo so it would be easier to pass data between the intent extension and the main app in the event someone wanted to add other things to Loop's Siri capabilities

[novalegra]

My (pretty simple) implementation to set overrides just looks for the preset's name (and lowercases the names when comparing to avoid capitalization-related difficulties). This will fail to distinguish between overrides if they have the same name but different capitalization.

[novalegra]

This is a rename that I made on the LoopKit level

[ps2]

In the project file, PRODUCT_BUNDLE_IDENTIFIER = "com..loopkit.Loop.Loop-Intent-Extension"; needs to change to use the MAIN_APP_BUNDLE_IDENTIFIER. Something like this: PRODUCT_BUNDLE_IDENTIFIER = "$(MAIN_APP_BUNDLE_IDENTIFIER).Loop-Intent-Extension";