Support using environment variables to adjust referral checks

[emerick]

QA requested supporting environment variables to adjust some of the referral checks that we make. In particular, we should add support for the following enviornment variables:

BRAVE_REFERRALS_DELETE_TIME - Number of seconds after which to delete the referral code (default is 90 days)
BRAVE_REFERRALS_CHECK_TIME - Number of seconds to continue checking for referrals (default is 30 days)

[emerick]

cc: @kjozwiak @srirambv

[kjozwiak]

Went through verification using the following build under macOS 10.14 x64 - PASSED

Brave	0.56.12 Chromium: 70.0.3538.77 (Official Build) (64-bit)
Revision	0f6ce0b0cd63a12cb4eccea3637b1bc9a29148d9-refs/branch-heads/3538@{#1039}
OS	Mac OS X

• launch brave via BRAVE_REFERRALS_SERVER=laptop-updates-staging.herokuapp.com (make sure you're using a referral code that's on stating and NOT production)
• wait 10min (this is what the staging server is set too) or you'll receive Referral is not ready, please wait at least 30 days if you do the below steps without waiting 10min
• after waiting 10min, relaunch using BRAVE_REFERRALS_SERVER=laptop-updates-staging.herokuapp.com BRAVE_REFERRALS_CHECK_TIME=5
• you should see a timestamp being created next to the promo_code, example: "promo_code":"MIM486","timestamp":"13186203834926030"
• confirmed with @aekeus that the referral was finalized on the backend.

Verified passed with macOS 10.12.6 x64 and the following build:

Brave	0.56.12 Chromium: 70.0.3538.77 (Official Build) (64-bit)
Revision	0f6ce0b0cd63a12cb4eccea3637b1bc9a29148d9-refs/branch-heads/3538@{#1039}
OS	Mac OS X

• launch brave via BRAVE_REFERRALS_SERVER=laptop-updates-staging.herokuapp.com (make sure you're using a referral code that's on stating and NOT production)
• wait 10min (this is what the staging server is set too) or you'll receive Referral is not ready, please wait at least 30 days if you do the below steps without waiting 10min
• after waiting 10min, relaunch using BRAVE_REFERRALS_SERVER=laptop-updates-staging.herokuapp.com BRAVE_REFERRALS_CHECK_TIME=5
• you should see a timestamp being created next to the promo_code, example: "promo_code":"LAU795","timestamp":"13186246996115100"
• confirmed with @aekeus that the referral was finalized on the backend.
• confirmed for YT site (LAU795), Twitch (ILI371), and other site (KJO580)

Verification passed on

Brave	0.56.12 Chromium: 70.0.3538.77 (Official Build) (64-bit)
Revision	0f6ce0b0cd63a12cb4eccea3637b1bc9a29148d9-refs/branch-heads/3538@{#1039}
OS	Windows

• launch brave via BRAVE_REFERRALS_SERVER=laptop-updates-staging.herokuapp.com (make sure you're using a referral code that's on stating and NOT production)
• wait 10min (this is what the staging server is set too) or you'll receive Referral is not ready, please wait at least 30 days if you do the below steps without waiting 10min
• after waiting 10min, relaunch using BRAVE_REFERRALS_SERVER=laptop-updates-staging.herokuapp.com BRAVE_REFERRALS_CHECK_TIME=5
• you should see a timestamp being created next to the promo_code, example: "promo_code":"SGE795","timestamp":"13186223576927752"
• confirmed with @aekeus that the referral was finalized on the backend.
• confirmed for YT site (SGE795), Twitch (GSA362)