feat: Force readonly

[markphelps]

Gives us (and operators) to force the UI into read-only mode by setting the following:

experimental:
  filesystem_storage:
    enabled: true

storage:
  readOnly: true

In this next release we will be 'graduating' filesystem storage, so the experimental bit will not be required.

The goal of this feature is two fold:

1. To enable devs to be able to more easily test the UI in readonly mode (without requiring us to configure a readonly storage mode)
2. To allow operators to put Flipt in readonly mode even if backed by a database

For 2., more work will need to be done either at the API or ideally storage layer to reject all write requests, as currently this change only affects the UI so users could still make API requests or modify data via import/export even if storage: readonly: true

This also modifies the badge in the UI to show an icon based on what storage mode is in use:

• database
• local
• git
• object

(images in order)

[GeorgeMac]

Super neat

[codecov]

Codecov Report

Merging #1973 (e4b1736) into main (fddcf20) will increase coverage by 0.02%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##             main    #1973      +/-   ##
==========================================
+ Coverage   71.13%   71.16%   +0.02%     
==========================================
  Files          67       67              
  Lines        6572     6578       +6     
==========================================
+ Hits         4675     4681       +6     
  Misses       1634     1634              
  Partials      263      263              

Files Changed	Coverage Δ
internal/cmd/auth.go	0.00% <0.00%> (ø)
internal/config/storage.go	81.42% <100.00%> (+1.74%)	⬆️

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