Change default branch from 'master' to 'main' (#1589)

The default branch has been changed. This PR cleans up references to
`master` and fixes links that would otherwise be broken.

For more background see the following references:
* https://www.independent.co.uk/life-style/gadgets-and-tech/news/github-master-slave-slavery-whitelist-language-inclusive-a9568576.html
* https://tools.ietf.org/id/draft-knodel-terminology-00.html

docs/posts/launch.md

@@ -25,7 +25,7 @@ You should see your web browser open with the following
 You can also launch from a URL directly like this:
 
 ```
-cellxgene launch https://github.com/chanzuckerberg/cellxgene/blob/master/example-dataset/pbmc3k.h5ad
+cellxgene launch https://github.com/chanzuckerberg/cellxgene/blob/main/example-dataset/pbmc3k.h5ad
 ```
 
 Support for S3 and GCS is not enabled by default. If you wish to directly access S3 or GFS, install one or both of the following packages:

docs/posts/methods.md

@@ -17,7 +17,7 @@ cellxgene looks for embeddings (e.g., tSNE, UMAP, PCA, spatial coordinates) in `
 ## Differential expression
 
 We're actively working on how to improve differential expression within the app.
-**N.B.: the [current implementation](https://github.com/chanzuckerberg/cellxgene/blob/master/server/app/scanpy_engine/diffexp.py#L40) assumes normally distributed values on a linear scale.**
+**N.B.: the [current implementation](https://github.com/chanzuckerberg/cellxgene/blob/main/server/app/scanpy_engine/diffexp.py#L40) assumes normally distributed values on a linear scale.**
 
 Currently, we use a [Welch's _t_-test](https://en.wikipedia.org/wiki/Welch%27s_t-test), which assumes that the two populations are each normally distributed, but may have unequal variance. We use a two-sided t-test against the null hypothesis that the two populations have **equal** means. P-values are adjusted with the [Bonferroni corrrection](https://en.wikipedia.org/wiki/Bonferroni_correction).