Consistently use image paths relative to _media directory as described in the documentation

[caendesilva]

The docs explain that images using the Image object should be in the _media directory when they are referenced by a local path, but we are not consistently doing this. This PR fixes that.

Will fix the following:

• Fix Path property in Image model should be relative to media directory #359
• Fix Add toString method to Image model to get the link #370
• Fix Blog post OpenGraph images must be resolved relatively #374

[codecov]

Codecov Report

Merging #375 (762ffd0) into master (38d77bb) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master      #375   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       861       867    +6     
===========================================
  Files             93        93           
  Lines           2129      2142   +13     
===========================================
+ Hits            2129      2142   +13     

Impacted Files	Coverage Δ
...k/src/Actions/FindsContentLengthForImageObject.php	100.00% <100.00%> (ø)
packages/framework/src/Models/Image.php	100.00% <100.00%> (ø)
...ckages/framework/src/Models/Pages/MarkdownPost.php	100.00% <100.00%> (ø)

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