Change monthly contribution/payment sequencing

[davidtemkin]

When the time comes to do monthly contributions, the sequence should be as follows:

• Pay each monthly tip in sequence until there is insufficient funds. Never pay a partial monthly tip.
• After either (a) all monthly tips have been paid, or (b) there are not enough funds to handle the next monthly tip, move to handling Auto-Contribute
• For Auto-Contribute, take the remaining funds, up to the user-specified monthly A-C budget, and contribute that amount to verified publishers only, in proportion to the attention they received.

[GeetaSarvadnya]

Verification passed on

Brave	0.70.120 Chromium: 78.0.3904.70 (Official Build) (64-bit)
Revision	edb9c9f3de0247fd912a77b7f6cae7447f6d3ad5-refs/branch-heads/3904@{#800}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified the test plan from Adds contribution queue brave-core#3658
• Verified the STR's from Adds contribution queue brave-core#3658 (comment)
• Encountered receiving Insufficient Funds/AC can't be completed messages incorrectly - follow up to 6288 #6546 and Left sig != right sig. Signature check failure. message occurs very frequently - follow up to 6288 #6545
• Additional scenarios executed and are working as expected, in each scenarios 30 BAT grants is claimed

Verified passed with

Brave	0.70.120 Chromium: 78.0.3904.70 (Official Build) (64-bit)
Revision	edb9c9f3de0247fd912a77b7f6cae7447f6d3ad5-refs/branch-heads/3904@{#800}
OS	macOS Version 10.13.6 (Build 17G5019)

• Verified the test plan from Adds contribution queue brave-core#3658
• Verified the STR's from Adds contribution queue brave-core#3658 (comment)
• Encountered receiving Insufficient Funds/AC can't be completed messages incorrectly - follow up to 6288 #6546 and Left sig != right sig. Signature check failure. message occurs very frequently - follow up to 6288 #6545

Verification passed on

Brave	0.70.120 Chromium: 78.0.3904.70 (Official Build) (64-bit)
Revision	edb9c9f3de0247fd912a77b7f6cae7447f6d3ad5-refs/branch-heads/3904@{#800}
OS	Ubuntu 18.04 LTS

• Verified the test plan from Adds contribution queue brave-core#3658
• Verified the STR's from Adds contribution queue brave-core#3658 (comment)
• Encountered receiving Insufficient Funds/AC can't be completed messages incorrectly - follow up to 6288 #6546 and Left sig != right sig. Signature check failure. message occurs very frequently - follow up to 6288 #6545