Add new Monthly Contribution banner for publishers

[ryanml]

Design:

Prompt within the panel:

Test Plan:

1. Enable Brave Rewards, Deposit BAT
2. Navigate to a verified publisher, hit the rewards icon
3. Click the Send a Tip button
4. Confirm that the regular tip banner shows up as expected, with no option to check monthly.
5. Complete the tip and confirm that it processes via the tip table
6. Navigate to the above publisher (or another verified publisher)
7. Click the Send button in the Monthly Contribution row
8. Confirm the new monthly tip banner variant appears
9. Complete the contribution and verify that it shows up as a monthly contribution
10. Navigate back to the last publisher, hit the rewards icon
11. Confirm that you can change the monthly contribution amount via the panel

[GeetaSarvadnya]

Verification passed on

Brave	1.1.4 Chromium: 78.0.3904.97 (Official Build) beta (64-bit)
Revision	021b9028c246d820be17a10e5b393ee90f41375e-refs/branch-heads/3904@{#859}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified the test plan mentioned in the issue.

Verification passed on

Brave	1.1.4 Chromium: 78.0.3904.97 (Official Build) beta (64-bit)
Revision	021b9028c246d820be17a10e5b393ee90f41375e-refs/branch-heads/3904@{#859}
OS	Ubuntu 18.04 LTS

• Verified the test plan from the description
  
  
  
  
  

Verified passed with

Brave	1.1.5 Chromium: 78.0.3904.97 (Official Build) beta (64-bit)
Revision	021b9028c246d820be17a10e5b393ee90f41375e-refs/branch-heads/3904@{#859}
OS	macOS Version 10.13.6 (Build 17G5019)

• Verified test plan from description
• Confirmed if you change Monthly Contribution value on panel that change is reflected in Monthly Contribution panel on brave://rewards
• Confirmed setting Monthly Contribution in panel to 0.0 BAT removed the monthly tip
• Confirmed still able to remove monthly tip by clicking on the trash can icon on the Monthly Contribution panel in brave://rewards
• Confirmed custom tip banner works with Monthly Contribution banner.