Timeline line: Replace 1px image with CSS
Summary:
Replace the thin grey timeline line created via a 1px image file with modern CSS.
According to https://caniuse.com/?search=linear-gradient this should be long supported, except for Safari versions before 2022 which may have some problems (or not).
Closes T16202
Refs T15056
Test Plan: Go to a page which exposes the vertical timeline line, e.g. a Maniphest task, and look at it. (Note that depending on browser zoom levels there might be small differences.)
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16202, T15056
Differential Revision: https://we.phorge.it/D26229
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