add button to export issues to Excel

[tpokorra]

This PR adds a button to the issues list, and when you click on it, the currently displayed issues will be downloaded as an Excel file.

fixes #29906

Screenshot after adding the button "Export to Excel":

[GiteaBot]

@tpokorra I noticed you've updated the locales for non-English languages. These will be overwritten during the sync from our translation tool Crowdin. If you'd like to contribute your translations, please visit https://crowdin.com/project/gitea. Please revert the changes done on these files. 🍵

[silverwind]

The button is far too prominent and there should only ever be one primary button on a page indicating the most likely action the user will take.

I recommend making a right-aligned "..." button on the right side of "Sort" and putting it in there. Check repo frontpage on how to implement this "..." menu using tippy.js.

[tpokorra]

I recommend making a right-aligned "..." button on the right side of "Sort" and putting it in there. Check repo frontpage on how to implement this "..." menu using tippy.js.

Thanks for the hint @silverwind

I guess you are refering to initClonePanelButton in https://github.com/go-gitea/gitea/blob/main/web_src/js/features/repo-common.ts#L133 which uses Tippy.
and https://github.com/go-gitea/gitea/blob/main/templates/repo/clone_panel.tmpl#L6

I tried to find the place where I should add that new button.
This is where the sort dropdowns are defined: https://github.com/go-gitea/gitea/blob/main/templates/repo/issue/filter_list.tmpl#L121
this is included here: https://github.com/go-gitea/gitea/blob/main/templates/repo/issue/filters.tmpl#L22

I will have to continue trying to figure this out in the coming days.