Harden favicon loading logic to fix crash when loading certain web pages

[kylehickinson]

Description:

Currently the favicon loaders can load PDF files if the URL response when loading /favicon.ico actually returns pdf data. This is a result of SDWebImage being able to decode it and return an "image" as a result instead of throwing an error.

There are multiple points of hardening that should happen:

• Update metadata retrieval to append favicon.ico to the base domain and not the URL. In the case of the example PDF, the url ends up being https://static1.squarespace.com/static/57338e2c04426238cff175c6/t/60c9462f924a473dc95dc850/1623803439283/ListaDeVinos...pdf/favicon.ico which is incorrect, it should have been attempting to fetch https://static1.squarespace.com/favicon.ico
• Update favicon loader to reject PDF's as an acceptable favicon image. (Combination of rejecting Content-Type: application/pdf response headers and possibly file header byte checking)
• Impose favicon size limitation when inserting into Favicon DB table
• Purge any Database records of favicons who's sizes exceed 1024x1024 to ensure any users who have already cached a large image or non-image be removed and re-downloaded correctly

Steps to Reproduce

1. Visit https://static1.squarespace.com/static/57338e2c04426238cff175c6/t/60c9462f924a473dc95dc850/1623803439283/ListaDeVinos...pdf
2. If it doesn't crash immediately (due to your device having sufficient memory), tap on tab tray button

Actual result: Crashes
Expected result: Shouldn't crash
Reproduces how often: Always
Brave Version: Live (1.27.1)
Device details: Depending on the devices memory limitations it may crash while downloading the favicon or when displaying it

[srirambv]

Verification passed on iPhone XR with iOS 13.5 running 1.29(21.07.20.16)

• Verified steps from issue description
• Verified no browser crash when opening the page or opening tab tray
• Verified on both normal and private tabs

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Verification passed on iPhone 7+ with iOS 14.5.1 running 1.29(21.07.20.16)

• Verified steps from issue description
• Verified no browser crash when opening the page or opening tab tray
• Verified on both normal and private tabs

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Verification passed on iPad Pro with iOS 14.6 running 1.29(21.07.20.16)

• Verified steps from issue description
• Verified no browser crash when opening the page or opening tab tray
• Verified on both normal and private tabs