Feat - url for dark version of placeholder images

[nico3333fr]

Short description of what this resolves:

Missing support of dark mode version for placeholder illustrations.

Changes proposed in this pull request:

• added urlDarkVersion property (not mandatory, in some cases, the image could fit in both cases ^^)
• if specified, will be displayed
• added a class hide-on-darkmode in case of dark version (to hide the "light" version)

Snapshot

[EpokK]

I don't think it's a good idea to add a property for the dark theme like that. I prefer if we could get this information via a React hook config or css directly. I suppose, you already check the second choice. Check with @econdepe how to know if dark is applied.

[nico3333fr]

Yes, we already had this discussion with @econdepe, this rely on CSS.

[mmso]

I'm ok with this for now and we can think of how to improve it later

[econdepe]

We introduced a dummy class in in the PR #537 for an unrelated issue (Nico needed it in the subscription flow). Since the class is there, it does make sense to re-use it as in this PR, but I agree a React hook would look nicer.

[mmso]

I think this suggestion looks slightly better than this PR though:

// somewhere
export const getLightOrDark = (light, dark) => {
    return document.body.classList.contains('isDarkMode') ? dark : light;
};
// elsewhere
<IllustrationPlaceholder
    url={getDarkOrLight(lighImageImport, darkImageImport)}
    ...
/>

I think it's as fast as doing this, and then we can refactor with the ThemeProvider later

[econdepe]

Sorry about the force-pushes (the first one was unintentional...). The helper is available in ProtonMail/proton-shared#114
This PR now just moves a constant to proton-shared