Fetch oldest authored_at correctly instead of oldest authored_at per page

[albertchae]

Fixes https://github.com/rubygems/rubygems.org/issues/4448

Previously, the "since date" was using oldest version.authored_at for a given page. This change fixes it to get the oldest version.authored_at in the database. Because authored_at depends on either built_at or created_at, I added a scope to replicate this behavior via SQL and get the minimum. This should only be 1 additional query for one column.

I modified existing tests to check for the "versions since" text and also have 2 pages of actual content.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1d33ca5) 97.15% compared to head (c26f464) 97.15%. Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4460   +/-   ##
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  Coverage   97.15%   97.15%           
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  Files         391      391           
  Lines        8259     8264    +5     
=======================================
+ Hits         8024     8029    +5     
  Misses        235      235           

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[albertchae]

@simi thanks for all the reviews and comments so far. Was there anything else you wanted me to address for this PR?