fetch-issues.js: Use dynamic date for Netlify preview deploy pre-fetching

[li-boxuan]

Netlify deploy for pull requests has 15 min time limit.

gh-board/script/fetch-issues.js

Line 17 in 38a94ce

'2018-04-01T00:00:00Z';

uses a hard-coded date.

This should be changed into a dynamic date, e.g. 3 months ago.

[bhawesh96]

Can I work on this issue?