Add description meta tag to transition page

[xming13]

Problem

Closes #546

Solution

Implement getLongUrlAndDescription to retrieve long url and description for a given short url. It tries to fetch from redis cache first before falling back to fetching from database.
Injecting description meta tag to transition page is done in transition-page template.

Note:
Changes to the format of existing cache value

Old
cache key: short url
cache value: long url

New
cache key: short url
cache value: JSON.stringify({ longUrl: longUrl, description: description })

This change is backward compatible. Old cache value (ie. long url) will fail when it is parsed as json string, throwing InvalidFormatError. System will then read from database and update the cache value to the new format. Subsequent read from redis will be parsed successfully based on the new format.

Before & After Screenshots

AFTER:

Tests

Refer to the changes related to tests in this commit.

Deploy Notes

Monitor for backward compatibility in case anything goes wrong unexpectedly.

[LoneRifle]

lgtm