Ensure default avatars get alt text

[dkotter]

Description of the Change

In #127 we ensured that if an image that was set as an avatar had alt text associated with it, that alt text would be output. But if someone doesn't have a custom avatar set and instead we use a default avatar, that still doesn't output alt text.

This PR fixes that by:

1. Setting default alt text that will be used if no alt text can be found (defaulted to Avatar photo with a new filter around that so it can be changed)
2. If the default avatar being used was uploaded (instead of a default gravatar image) we try and pull the alt text from that image, if it exists. If not, we use the default mentioned above

Closes #146

How to test the Change

1. Ensure you're using a theme that outputs avatars
2. Set a default avatar to be one of the Gravatar options
3. Go to a post that was authored by someone without a custom avatar set
4. Inspect the source of the avatar and note the alt text should be showing Avatar photo
5. Change the default avatar to be an uploaded image that has alt text set
6. Inspect the source again and note the alt text should now be what was set on that image
7. If desired, test the simple_local_avatars_default_alt filter to ensure you can change the default alt text

Changelog Entry

Added - If a default avatar image is used, ensure that outputs alt text. This will either be default text (Avatar photo) or the alt text from the uploaded default image

Credits

Props @dkotter

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[faisal-alvi]

• ✅ Code LGTM
• ✅ Working as expected.
• ✅ Issue re-generated in the develop branch:

image link

• ✅ Default alt text:

image link

• ✅ Alt text from image:

image link