resourceloader: Reduce width of module hash from 7 chars to 5

resourceloader: Reduce width of module hash from 7 chars to 5

In a nut shell:

* We very often (52% of modules on enwiki) pad the hash with a zero,
  which means the amount of bits we currently compute already fit in
  6 characters already for most modules. For some modules (3%) we
  even padded two zeroes.

* For the (now documented) use cases, the space of 78 Giga
  (78 billion, or 78 milliard) seems more than we need. The space of
  60 million should be enough.

  This follows-up dfd046412f from 2016, which previously shortened the hash
  down from 8 chars of base 64 (or 12 chars of hex) to 7 chars of base 32.
  Before that change, the space was 281 Tera (64^8, or 16^12).

For more details see the added inline comment for ResourceLoader::makeHash,
and also the data at <https://phabricator.wikimedia.org/T229245>.

Bug: T229245
Change-Id: I9ad11772a33b3a44cb625275b1d7353e1393ee49