Favicon fix

[HammadTheOne]

This PR fixes the favicon issue described in plotly/dash-enterprise-docs#636. It adds a default Dash favicon and updates the tag to be set when no favicon.ico file is provided in the assets directory, in parity with Dash for Python. It also removes a redundant codeblock that may have been left behind from an earlier PR.

[rpkyle]

@HammadTheOne Just two minor comments -- I'd test this with a deployed app before merging, but assuming that works (and an app with another, non-default favicon displays properly), I'd say we're 👌.

Is there a test on the Python side for this handler? I feel like it wouldn't hurt to add a really basic test to lock in the desired behaviour here... since this is what led to us missing this issue until @VeraZab's wizardry identified it for us.

Here's an example of a recent test I wrote that introspects the tags and ensures that they're present; there's probably a better way to do it but this is what I used 😅

dashR/tests/testthat/test-attributes.R

Lines 39 to 56 in edbcc04

	response_with_attributes <- app$server$test_request(request_with_attributes)
	tags_by_line <- lapply(strsplit(response_with_attributes$body, "\n "), function(x) trimws(x))[[1]]
	stylesheet_hrefs <- grep(stylesheet_pattern, tags_by_line, value = TRUE)
	script_hrefs <- grep(script_pattern, tags_by_line, value = TRUE)
	expect_equal(
	stylesheet_hrefs,
	"<link href=\"https://codepen.io/chriddyp/pen/bWLwgP.css\" hreflang=\"en-us\" rel=\"stylesheet\">"
	)
	expect_equal(
	script_hrefs,
	c("<script src=\"https://www.google-analytics.com/analytics.js\"></script>",
	"<script src=\"https://cdn.polyfill.io/v2/polyfill.min.js\"></script>",
	"<script src=\"https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.10/lodash.core.js\" integrity=\"sha256-Qqd/EfdABZUcAxjOkMi8eGEivtdTkh3b65xCZL4qAQA=\" crossorigin=\"anonymous\"></script>"
	)
	)