Refactor FXIOS-9833 General favicon related improvements to SiteImageModel and related classes

[ih-codes]

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:bulb: Description

Note to self: I have the merge destination currently set to my other favicon refactor branch so the changes can be more clearly reviewed. But merge in FXIOS-9667 fix first.

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Apologies for the long PR. 🙈 Most of the changes are naming improvements. Half the changes are in the unit tests.

General refactoring for some things I noticed while finishing up the favicon rework:

• Simplified SiteImageModel so it won't be possible to get empty cache key strings, empty resource URL strings, String and URL representation of the same data (siteURL, siteURLString), etc.
• Generalized SiteImageModel so it has a single resource URL rather than separate hero and favicon URLs (it only has one type after all...) and a non-optional siteURL (doesn't make sense to have a SiteImageModel if no site)
• Moved cache key generation to the SiteImageModel and vastly simplified the logic (seemed unnecessarily complicated). Made sure all code needing cache keys calls this method.
• Updated "site" param naming to be "model" when passing around a SiteImageModel (site is misleading)
• Methods prefixed with "getURL" or "getImage" that actually returned a SiteImageModel now return what they say they're returning (e.g. a URL, a UIImage, etc.)
• Instead of passing around .absoluteString of several URLs, just pass around the URL to avoid redundant conversions
• Updated some documentation that referred to old code
• Remove optionals where there was no need to be optional
• Add a guard so we don't try to cache empty faviconURLs from the Tab metadata update didSet after loading a webpage
• Prefer modern concurrency to execute UI updates on the main thread instead of GCD (especially when inside a Task)
• Removed unnecessary do/catch blocks, or moved them to be more contextual with what they were wrapping
• Unit test updates. SiteImageHandlerTests no longer test the exact same things as ImageHandlerTests (hence deleted tests).
• Added a few FIXMEs for discussion

:pencil: Checklist

You have to check all boxes before merging

• [x] Filled in the above information (tickets numbers and description of your work)
• [x] Updated the PR name to follow our PR naming guidelines
• [x] Wrote unit tests and/or ensured the tests suite is passing
• [ ] When working on UI, I checked and implemented accessibility (minimum Dynamic Text and VoiceOver)
• [ ] If needed, I updated documentation / comments for complex code and public methods
• [ ] If needed, added a backport comment (example @Mergifyio backport release/v120)