add notification for monthly tip contribution

[LaurenWags]

Description

Currently we do not have a notification for when monthly tips are contributed. This may not be needed if auto-contribute is turned on (since tips and a-c are contributed at relatively the same time), however when auto contribute is off, it might be nice to have a notification that monthly tip contributions have processed.

cc @mandar-brave for consideration, additional information/details.

[btlechowski]

Verification passed on

Brave	0.64.60 Chromium: 74.0.3729.91 (Official Build) beta (64-bit)
Revision	03844ed83e02b8add3f4b9cb859a7108d55b2e4d-refs/branch-heads/3729@{#860}
OS	Windows 10 OS Build 17134.523

Used test plan from brave/brave-core#1959

Logged #4236

Verification passed on

Brave	0.64.62 Chromium: 74.0.3729.108 (Official Build) beta (64-bit)
Revision	daaff52abef89988bf2a26091062160b1482b108-refs/branch-heads/3729@{#901}
OS	Linux

• Verified the test plan from Adds notification for when tips are processed brave-core#1959

Verified passed with

Brave	0.64.72 Chromium: 74.0.3729.131 (Official Build) beta(64-bit)
Revision	518a41c1fa7ce1c8bb5e22346e82e42b4d76a96f-refs/branch-heads/3729@{#954}
OS	Mac OS X

• Verified test plan from Adds notification for when tips are processed brave-core#1959