Security fix: Added nonce check to CSV importer actions

[timbocode]

woocommerce/woocommerce@cabf9de

See also #214

Files affected:
includes/admin/class-wc-admin-importers.php
includes/admin/importers/class-wc-product-csv-importer-controller.php

[bahiirwa]

Changes for email 3c35038 should not be in this PR. Could you remove 3c35038?

[codecov-io]

Codecov Report

Merging #221 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #221      +/-   ##
============================================
- Coverage     40.83%   40.82%   -0.01%     
  Complexity    13447    13447              
============================================
  Files           367      367              
  Lines         51275    51280       +5     
============================================
- Hits          20938    20936       -2     
- Misses        30337    30344       +7

Impacted Files	Coverage Δ	Complexity Δ
...rters/class-wc-product-csv-importer-controller.php	12.57% <0%> (-0.12%)	69 <0> (ø)
includes/admin/class-wc-admin-importers.php	0% <0%> (ø)	42 <0> (ø)	⬇️
...shipping/flat-rate/includes/settings-flat-rate.php	97.95% <0%> (-2.05%)	0% <0%> (ø)
includes/class-wc-tax.php	79.67% <0%> (-0.28%)	130% <0%> (ø)
includes/class-wc-update-client.php	0% <0%> (ø)	64% <0%> (ø)	⬇️

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[timbocode]

Testing

• Manually imported properly formated CSV file. OK.
• Manually tested the CSV import using a badly formatted CSV file. It successfully rejected the file.

[nylen]

I compared the changes here against woocommerce/woocommerce@cabf9de and this looks good.

[timbocode]

Changes for email 3c35038 should not be in this PR. Could you remove 3c35038?

@bahiirwa can you review please?

[bahiirwa]

Tested the branch locally and imported a number of assets using a dummy csv and txt. So all looks fine for user endpoint.