Publisher banner: incorrect letter used thank you banner

[mandar-brave]

Test Plan

brave/brave-core#1410

When tipping unverified/verified YT channel, the thank you screen shows a “Y” in a circle, vs the first letter of the channel.
Need to use the first letter of the channel consistently across tip-banner and thank-you-banner.

cc @davidtemkin @NejcZdovc

[jonathansampson]

This applies to verified publishers too:

[btlechowski]

Verification passed on

Brave	0.60.28 Chromium: 72.0.3626.96 (Official Build) beta (64-bit)
Revision	84098ee7ef8622a9defc2ef043cd8930b617b10e-refs/branch-heads/3626@{#836}
OS	Windows 7

For some verified publishers there is no icon at all. Logged: #3364

Unverified publishers are shown with the proper letter.
Channel URL: https://www.youtube.com/channel/UCwRMYIQ89FhDouMEASM5uAA
Channel name: MrShoptaw
Banner:

Verification passed on

Brave	0.60.34 Chromium: 72.0.3626.109 (Official Build) beta(64-bit)
Revision	fae8db7ab9280fa6704a59980263c804f809ebd5-refs/branch-heads/3626@{#857}
OS	Linux

• Unverified pubs shown with first letter for both media and non media publishers

Verification PASSED on macOS 10.14.2 x64 using the following build:

Brave	0.60.44 Chromium: 72.0.3626.109 (Official Build) (64-bit)
Revision	fae8db7ab9280fa6704a59980263c804f809ebd5-refs/branch-heads/3626@{#857}
OS	Mac OS X

Went through the following cases and ensured that the correct icon/letter is being used under the Thank You banner when tipping:

Unverified YT Channels - **PASSED**

* https://www.youtube.com/channel/UCCatR7nWbYrkVXdxXb4cGXw
* https://www.youtube.com/channel/UCyRhIGDUKdIOw07Pd8pHxCw
* https://www.youtube.com/user/boardgamegeektv

Verified YT Channels - **PASSED**

* https://www.youtube.com/user/sxephil
* https://www.youtube.com/channel/UCazMm3tOCkYrIGE_17j0mVg
* https://www.youtube.com/user/vice

Unverified Websites - **PASSED**

* https://www.reddit.com/
* https://twitter.com/
* https://facebook.com/

Verified Websites - **PASSED**

* https://clifton.io/
* https://coinpaprika.com/
* bodybuilding.com/

Unverified Twitch Channels - **PASSED**

* https://www.twitch.tv/shipwreckfive
* https://www.twitch.tv/timthetatman
* https://www.twitch.tv/tfue

Verified Twitch Channels - **PASSED**

* https://www.twitch.tv/psherotv
* https://www.twitch.tv/bwana
* https://www.twitch.tv/themattayahu