Add promotional card for Gemini in Brave Rewards settings page

[emerick]

Add a promotional card for Gemini in the Brave Rewards settings (brave://rewards) in the bottom right, where the TAP Network card typically goes (bottom right corner).

[stephendonner]

Verified PASSED using

Brave	1.29.50 Chromium: 92.0.4515.131 (Official Build) nightly (x86_64)
Revision	6b8d6c56ce21e38a72f7c4becb5abc1fa5134f29-refs/branch-heads/4515@{#1933}
OS	macOS Version 11.5.1 (Build 20G80)

Confirmed:

1. new profile has both Gemini and Tap
2. confirmed the card URL is https://www.gemini.com/brave
3. clicking on the x removes Gemini, which persists after restart
4. clicking on the x removes Tap, which persists after restart
5. upgrading from 1.27.109 adds Gemini (Gemini at top and Tap, below it, displayed)
6. upgrading from 1.27.109 with Tap removed adds Gemini

example	example	example	example	example	example

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Verification passed on

Brave | 1.29.60 Chromium: 92.0.4515.131 (Official Build) beta (64-bit)
-- | --
Revision | 6b8d6c56ce21e38a72f7c4becb5abc1fa5134f29-refs/branch-heads/4515@{#1933}
OS | Windows 10 OS Version 2009 (Build 19043.1165)

• Verified the test plan from Add promo card for Gemini brave-core#9623

1. new profile has both Gemini and Tap
2. confirmed the card URL is https://www.gemini.com/brave
3. clicking on the x removes Gemini
4. clicking on the x removes Tap

example	example

Verified both clean and upgraded profiles.

• Installed 1.28.102 and ensured only TAP card is shown in the brave://rewards page, upgraded the profile to 1.29.60 and ensured Gemini card is also shown along with TAP card in brave://rewrads page

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Verified PASSED using the testplan from brave/brave-core#9623 with

Brave	1.29.68 Chromium: 92.0.4515.159 (Official Build) beta (64-bit)
Revision	0185b8a19c88c5dfd3e6c0da6686d799e9bc3b52-refs/branch-heads/4515@{#2052}
OS	Linux

1. new profile has both Gemini and Tap
2. confirmed the card URL is https://www.gemini.com/brave
3. clicking on the x removes Gemini
4. clicking on the x removes Tap

example	example

Verified both clean and upgraded profiles.

• Installed 1.28.106 and ensured only TAP card is shown in the brave://rewards page, upgraded the profile to 1.29.68 and ensured Gemini card is also shown along with TAP card in brave://rewards page

[LaurenWags]

#17383 can be tested along side this issue.