Add contacts CSV export capability

[katafrakt]

This adds possibility to export contacts from a project or a segment to CSV, compatible with CSV import. It addresses #110.

I wasn't really sure where to put the buttons, so please advise for a better place.

Screenshots

Notes

The code in these two controllers look very similar. I wasn't sure if it makes sense to abstract it out and especially - where. This feels like highly controller-level code, so pushing it to a context does not feel right. There could be a shared codebase space for just controllers, but I didn't want to just create it without consultation.

[wmnnd]

This is awesome, thank you for creating this PR, @katafrakt! 🥳

Here are some small changes to the code I’d suggest:

1. In most interfaces in Keila, all action buttons are in the top row, so I suggest you put the export button on the contacts and segment pages up there as well, probably in the left-most position.
2. Like you’ve already mentioned, the code for both controllers is pretty much the same. I think it makes sense to create a helper module for centralizing this code. My suggestion would be adding KeilaWeb.ContactsCsvExport in lib/keila_web/helpers with a function stream_csv_response/3 that takes as its first argument Plug.Conn and the arguments for Contacts.stream_project_contacts/2 as its second and third argument.
3. I also recommend you move the chunk configuration away from being directly under the :keila key to something like config :keila, KeilaWeb.Helpers.CsvExport, chunk_size: 100

[katafrakt]

@wmnnd I made the changes you suggested.

I'm not 100% sure about the location of the download button in the segment page. It is very detached from the contacts list. And, until the segment will be saved, it will not reflected the changes done by the user. Now I'm thinking that maybe that button should be even placed here?

Not sure though, it's your project and you have the vision for it, I'm just sharing some thoughts.

[wmnnd]

@katafrakt Can you give me write permissions to your branch so I can adjust the button positions? Then I’ll merge your PR 😃

[katafrakt]

@wmnnd you should already have it. At least that's how I always understood this option.

[wmnnd]

Ah, you’re right. Looks like this just didn’t work with Github’s github.dev editor.

[wmnnd]

Thank you for your contribution, @katafrakt! I have just released a new version that includes mainly your new feature 🥳