Create a repository for commonly-used licenses

[fmarier]

#6809 introduces a number of copies of the same licenses in our repository.

Some licenses are almost always customized (e.g. MIT, BSD), but others (MPL, GPL, Unlicense, CC, OFL, Apache) are always exactly the same. For the latter, we could create a new src/brave/common-licenses/ and refer to files in there from the relevant README.chromium files.

[btlechowski]

Verification passed on

Brave	1.3.38 Chromium: 78.0.3904.108 (Official Build) nightly (64-bit)
Revision	4b26898a39ee037623a72fcfb77279fce0e7d648-refs/branch-heads/3904@{#889}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#4000

Examples:

[GeetaSarvadnya]

@fmarier I can see the following items in brave://credits. Except for these 4 items, I don't have remaining licenses in Windows 10 x64. Is this platform-specific or some items are missing in windows?

• PROCHLO (Apache-2.0)
• HTTPS Everywhere (GPL-2.0)
• Muli (OFL-1.1)
• Poppins (OFL-1.1)

[LaurenWags]

Verified passed with

Brave	1.3.88 Chromium: 79.0.3945.117 (Official Build) beta (64-bit)
Revision	04f0a055010adab4484f7497fbfdbf312c307f1d-refs/branch-heads/3945@{#1019}
OS	macOS Version 10.14.6 (Build 18G103)

• Verified test plan from Reuse common licenses in copyright statements  brave-core#4000
• Note, only PROCHLO, Brave HTTPS Everywhere, Muli, and Poppins are available on brave://credits without having to expand anything
• You need to expand Brave Ad Block or Brave Local Data to see the remainder of the licenses from Reuse common licenses in copyright statements  brave-core#4000

Verification passed on

Brave	1.3.87 Chromium: 79.0.3945.117 (Official Build) beta (64-bit)
Revision	04f0a055010adab4484f7497fbfdbf312c307f1d-refs/branch-heads/3945@{#1019}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified test plan from Reuse common licenses in copyright statements  brave-core#4000
• Note, only PROCHLO, Brave HTTPS Everywhere, Muli, and Poppins are available on brave://credits without having to expand anything
• You need to expand Brave Ad Block or Brave Local Data to see the remainder of the licenses from Reuse common licenses in copyright statements  brave-core#4000