Do not add HTTPS to links if protocol is declared

[lemker]

Current linking behavior seems to disregard protocols other than HTTP or HTTPS. For instance, an STSSYNC link will break the link when https:// is prepended to the link

To Reproduce
Steps to reproduce the behavior:

1. Create a new link using a protocol other than http/https, for example stssync://
2. Outline will add https:// to the link, breaking the protocol (https://stssync://)

Expected behavior
Leave stssync:// the way it is.

Screenshots

Outline (please complete the following information):

• Install: getoutline.com

Desktop (please complete the following information):

• OS: Windows 10 Pro
• Browser: Chromium Edge