Delete formatted images after image upload

This change fixes an issue where old/existing formatted images are used
when a filename is reused (by overwrite or by coincidence), regardless
of if the file contents have changed. To users this mainly manifests
as a file overwrite appearing not to work; the thumbnails in the CMS
show the original image until regeneration is forced.

Calling Image::deleteFormattedImages() after image upload ensures that
no stagnant formatted images will be used.
Fixed reference to the templates documentation
Merge pull request #3023 from camfindlay/patch-5

Fixed reference to the templates documentation
Returning response from doChangePassword()
Merge pull request #3024 from silverstripe-iterators/pulls/changepw-return

Returning response from doChangePassword()
API Security.remember_username to disable login form autocompletion
Merge pull request #3020 from tractorcow/pulls/3.1-autocomplete-username

API Security.remember_username to disable login form autocompletion
BUG Delete Character \x01
Merge pull request #3027 from stevie-mayhew/patch-3

FIX Delete Character \x01
MINOR Clarify DataQuery::where functionality

As per the doc type, this method will append a new filter, not *set* one.
Merge pull request #3036 from ss23/patch-2

MINOR Clarify DataQuery::where functionality
Merge pull request #3004 from stecman/delete-resampled-after-image-upload

Delete formatted images after image upload
BUG Load just one of each plugin.
Merge pull request #3039 from mateusz/plugins

BUG Load just one of each plugin.
Fixed small typo

ration changed to ratio
Merge pull request #3043 from martinduparc/patch-3

Fixed small typo
API HTTP 429 Allowed for use with rate limiting methods
Merge pull request #3047 from tractorcow/pulls/http-429

HTTP 429 Allowed for use with rate limiting methods
Wording fix to forms.md

Closes #3032