Refactoring theme helper

[escopecz]

Q	A
Branch?	3.0 for bug fixes
Bug fix?	yes
New feature?	no
Deprecations?	yes
BC breaks?	no
Automated tests included?	yes
Related user documentation PR URL	/
Related developer documentation PR URL	/
Issue(s) addressed	/

Description:

The goal was to enable to set directory name in a separate parameter to the copy() method. As generating it from the theme name may not be always what we want. Also to provide greater test coverage to the TestHelper. However this task was not possible without refactoring.

The ThemeHelper class was not fully testable as it was creating instances of Filesystem and Finder classes instead of providing those helpers via dependency injection so those could be mocked.

So I created 2 new services for those Symfony helpers and even created Mautic's own Filesystem helper by extending Symfony's Filesystem and adding the readFile() method to our implementation so we could use this one helper to manipulate files and folders. Symfony's version doesn't have that method for wrong reasons. Se need an abstraction above the native PHP methods that manipulate files so we could mock outputs in unit tests.

As we now have our own Filesystem helper I have deprecated Symfony's helper.

Steps to test this PR:

1. Load up this PR
2. Go to Themes
3. Clone a theme. Ensure it's visible in the list of Themes as well as in the Email form page.

[codecov]

Codecov Report

Merging #9316 (cbe7c89) into features (4ec27ae) will increase coverage by 0.01%.
The diff coverage is 56.09%.

@@              Coverage Diff               @@
##             features    #9316      +/-   ##
==============================================
+ Coverage       37.66%   37.67%   +0.01%     
- Complexity      33824    33826       +2     
==============================================
  Files            1965     1966       +1     
  Lines          104533   104535       +2     
==============================================
+ Hits            39368    39388      +20     
+ Misses          65165    65147      -18     

Impacted Files	Coverage Δ	Complexity Δ
app/bundles/CoreBundle/Helper/ThemeHelper.php	59.00% <54.05%> (+9.00%)	78.00 <6.00> (ø)
app/bundles/CoreBundle/Helper/Filesystem.php	75.00% <75.00%> (ø)	2.00 <2.00> (?)

[kuzmany]

Tested and works like before.
Code looks good and understand new possibilities
Unit test included.
Then approve it 👍

[RCheesley]

Tested on Mautibox and was able to install a new theme cloned from an existing one, and use it to create an email.