Adds a few small frontend fixes (#995)

* Add a few instructions to the readme on how to get `grocy` running locally

* Fix toggle for header-clock

I'm not 100% sure why, but with this change, the listener which calls
`CheckHeaderClockEnabled` will be invoked *before* the listener which
persists the setting.

If the setting is persisted before that, the clock doesn't show up when
enabling it in the settings-menu and appears/disappears in the exact
opposite way the setting is true/false.

* Allow replacing a product picture when removing it at first

Right now, a preview image of a product doesn't get updated when
pressing the delete-button at first and adding a new image the
upload-form which can be quite confusing for an end-user.

This patch allows to delete an image and add a new one in one go.

* Add `Save & return` button to product form

Same concept as for recipes: when pressing this button, the user will
stay at the form's site after saving.

* Removed unneeded class

* Revert "Add a few instructions to the readme on how to get `grocy` running locally"

This reverts commit 6ffad1d3c7.

Co-authored-by: Bernd Bestel <bernd@berrnd.de>

public/js/grocy_clock.js

@@ -1,4 +1,4 @@
-$("#show-clock-in-header").on("change", function()
+$(document).on("change", "#show-clock-in-header", function()
 {
 	CheckHeaderClockEnabled();
 });