Store last updated time in database

[swissspidy]

Description

This changes the updater so that after every string import Traduttore stores that time in the database.

Makes debugging a bit easier.

Fixes #114.

Checklist:

• [x] My code is tested.
• [x] My code follows the WordPress code style.
• [x] My code has proper inline documentation.

[codecov-io]

Codecov Report

Merging #120 into master will increase coverage by 0.19%. The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #120      +/-   ##
============================================
+ Coverage     77.66%   77.86%   +0.19%     
- Complexity      328      332       +4     
============================================
  Files            23       23              
  Lines           788      795       +7     
============================================
+ Hits            612      619       +7     
  Misses          176      176

Impacted Files	Coverage Δ	Complexity Δ
inc/Project.php	100% <100%> (ø)	48 <4> (+4)	:arrow_up:
inc/Updater.php	95% <100%> (+0.17%)	22 <0> (ø)	:arrow_down:

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[swissspidy]

There was a merge conflict, so now I need a new review approval 🙂