Use single config backup file when config.template is used

[oscarb]

Copy config.js to a single config.js-old file whenever a config.template is present, instead of using Date() as a suffix. Creating files with a timestamp suffix means that whenever Magic Mirror is restarted or recreated a new file is written into the config directory.

Benefits:

• Single backup file will avoid excessive writing of files
• Saves space and usage on SD cards.
• Makes the folder cleaner and easier to overview, compared to ending up with something like this as time goes on:
  

[codecov-commenter]

Codecov Report

Merging #3104 (c7e50e9) into develop (d832d79) will increase coverage by 0.04%.
The diff coverage is 0.00%.

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@@             Coverage Diff             @@
##           develop    #3104      +/-   ##
===========================================
+ Coverage    25.53%   25.57%   +0.04%     
===========================================
  Files           53       53              
  Lines        11420    11420              
===========================================
+ Hits          2916     2921       +5     
+ Misses        8504     8499       -5     

Impacted Files	Coverage Δ
js/app.js	79.88% <0.00%> (ø)

... and 1 file with indirect coverage changes

[rejas]

Thx for the PR, I am fine with it.
What dou you think @khassel as the OC (original coder)?