Fix alternate text for logo

Created by: AnandChowdhary

This is a minor issue after #692 (closed).

Current markup

The .navbar-brand link contains an image with the Sourcegraph logo, the first item in the header:

<img src="/sourcegraph/logo--light.svg" alt="Sourcegraph logo">

Correct markup

<img src="/sourcegraph/logo--light.svg" alt="Sourcegraph">

The alternate text (alt attribute) should be "Sourcegraph", not "Sourcegraph logo"

Why

Intuitively, the logo is a representation of the company name. When a screen reader reads the image, it should say "Sourcegraph / link", implying the home page of the company, rather than "Sourcegraph logo / link", implying a decorative image of the logo.

Specifically, Writing effective ALT text for images recommends:

Websites tend to vary in how they apply ALT text to logos. Some say, ‘Company name’, others ‘Company name logo’, and others describe the function of the image (usually a link back to the homepage) e.g. ‘Back to home’. Remember, ALT text should always describe the content of the image so the first example, alt="Company name", is probably the best.