Harmonize fileBased backends and reduce boilerplate

[LeaVerou]

We have three backends with fileBased = true.

backends/dropbox/index.js:

async upload (file, path) {
	if (this.ref.path) {
		path = this.ref.path.replace(/[^/]+$/, "") + path; // make upload path relative to existing path
	}
	else {
		path = path.startsWith("/") ? path : "/" + path;
	}

	const ref = this._getRef(path);
	await this.put(file, {ref, isUploading: true});

	return this.getURL(path);
}

backends/github/file/github-file.js:

async upload (file, path = this.ref.path) {
	let dataURL = await readFile(file);

	let base64 = dataURL.slice(5); // remove data:
	let media = base64.match(/^\w+\/[\w+]+/)[0];
	media = media.replace("+", "\\+"); // Fix for #608
	base64 = base64.replace(RegExp(`^${media}(;base64)?,`), "");
	path = this.ref.path.replace(/[^/]+$/, "") + path; // make upload path relative to existing path

	let fileInfo = await this.put(base64, {ref: path, isEncoded: true});
	return this.getFileURL(path, {sha: fileInfo.commit.sha});
}

async upload (file, {filename = file.name, folder} = {}) {
	const metadata = {
		name: filename
	};

	if (folder) {
		const folderId = GoogleDrive.#getFolderId(folder);

		if (folderId) {
			metadata.parents = [folderId];
		}
	}

	const res = await this.request("upload/drive/v3/files?&fields=webViewLink&uploadType=resumable", metadata, "POST", {responseType: "response"});
	const call = res.headers.get("Location");

	try {
		const fileInfo = await this.request(call, file, "PATCH");
		return fileInfo.webViewLink;
	}
	catch ({ error }) {
		if (error.code === 401) {
			await this.logout(); // Access token we have is invalid. Discard it.
			throw new Error(this.constructor.phrase("access_token_invalid"));
		}
		else if (error.code === 403) {
			throw new Error(this.constructor.phrase("no_upload_permission"));
		}

		throw new Error(error.message);
	}
}

1. We should harmonize these signatures.
2. We should not be duplicating code that deals with relative paths in the actual backends. If we know it’s file based, we can do that in a superclass. That's the whole point of the fileBased key.

I haven't looked at put() but I suspect the findings will be similar.